CHAPTER XVII

THE ORGANISM AS AN HISTORICAL BEING

"Of late the attempt to arrange genealogical trees involving hypothetical groups has come to be the subject of some ridicule, perhaps deserved. But since this is what modern morphological criticism in great measure aims at doing, it cannot be altogether profitless to follow this method to its logical conclusions. That the results of such criticism must be highly speculative, and often liable to grave error, is evident."

The quotation is from Bateson's paper of 1886, and it is symptomatic of the change which was soon to come over morphological thought. New interests, new lines of work, began to usurp the place which pure morphology had held so long.

This is accordingly a convenient stage at which to take stock of what has gone before, to consider the relation of evolutionary morphology to the transcendental and the Cuvierian schools of thought which preceded it, and to make clear what new element evolution-theory added to morphology.

The close analogy between evolutionary and transcendental morphology has already been remarked upon and illustrated in the last three chapters. We have seen that the coming of evolution made comparatively little difference to pure morphology, that no new criteria of homology were introduced, and that so far as pure morphology was concerned, evolution might still have been conceived as an ideal process precisely as it was by the transcendentalists. The principle of connections still remained the guiding thread of morphological work; the search for archetypes, whether anatomical or embryological, still continued in the same way as before, and it was a point of subordinate importance that, under the influence of the evolution-theory, these were considered to represent real ancestral forms rather than purely abstract figments of the intelligence. The law of Meckel-Serres was revived in an altered shape as the law of the recapitulation of phylogeny by ontogeny; the natural system of classification was passively inherited, and, by a petitio principii, taken to represent the true course of evolution. It is true that the attempt was made to substitute for the concept of homology the purely genetic concept of homogeny, but no inkling was given of any possible method of recognising homogeny other than the well-worn methods generally employed in the search after homologies.

There was a close spiritual affinity between the speculative evolutionists and the transcendentalists. Both showed the same subconscious craving for simplicist conceptions—the transcendentalists clung fast to the notion of the absolute unity of type, of the ideal existence of the "one animal," and the evolutionists did precisely the same thing when they blindly and instinctively accepted the doctrine of the monophyletic descent of all animals from one primeval form. Geoffroy persisted in regarding Arthropods as being built on the same plan as Vertebrates: Dohrn and Semper did nothing different when they derived both groups from an ancestor combining the main characters of both. The determination to link together all the main phyla of the animal kingdom and to force them all into a single mould was common to evolutionary and pre-evolutionary transcendentalists alike.

From the fact that all Metazoa develop from an ovum which is a simple cell, the evolutionists inferred that all must have arisen from one primordial cell. From the fact that the next step in development is the segmentation of the ovum, they argued that the ancestral Metazoa came into being through the division of the primal Protozoon with aggregation of the division-products. From the fact that a gastrula stage is very commonly formed when segmentation has been completed, they assumed that all germ-layered animals were descended from an ancestral Gastræa.

They quite ignored the possibility that a different explanation of the facts might be given; they seized upon the simplest and most obvious solution because it satisfied their overwhelming desire for simplification. But is the simplest explanation always the truest—especially when dealing with living things? One may be permitted to doubt it. It is easy to account for the structural resemblance of the members of a classificatory group, by the assumption that they are all descended from a common ancestral form; it is easy to postulate any number of hypothetical generalised types; but in the absence of positive evidence, such simplicist explanations must always remain doubtful. The evolutionists, however, had no such scruples.

Phylogenetic method differed in no way from transcendental—except perhaps that it had learnt from von Baer and from Darwin to give more weight to embryology. The criticisms passed by Cuvier and von Baer upon the transcendentalists and their recapitulation theory might with equal justice be applied to the phylogenetic speculations which were based on the biogenetic law. There was the same tendency to fix upon isolated points of resemblance and disregard the rest of the organisation. Thus, on the ground of a presumed analogy of certain structures to the vertebrate notochord, several invertebrate groups, as the Enteropneusta, the Rhabdopleura, the Nemertea, were supposed to be, if not ancestral, at least offshoots from the direct line of vertebrate descent. And if other points of resemblance could in some of these cases be discovered, yet no successful attempt was made to show that the total organisation of any of these forms corresponded with that of the Vertebrate type. With the possible exception of the Ascidian theory, all the numerous theories of vertebrate descent suffered from this irremediable defect, and none carried complete conviction.

In spite of the efforts of the evolutionists, as of those of the transcendentalists, the phyla or "types" remained distinct, or at best connected by the most general of bonds.

The close affinity of transcendentalists and evolutionists is shown very clearly in their common contrast in habits of thought with the Cuvierian school. It is the cardinal principle of pure morphology that function must be excluded from consideration. This is a necessary and unavoidable simplification which must be carried out if there is to be a science of pure form at all. But this limitation of outlook, if carried over from morphology to general biology becomes harmful, since it wilfully ignores one whole side of life—and that the most important. The functional point of view is clearly indispensable for any general understanding of living things, and this is where the Cuvierian school has the advantage over the transcendental—its principles are applicable to biology in general.

Geoffroy and Cuvier in pre-evolutionary times well typified the contrast between the formal and the functional standpoints. For Geoffroy form determined function, while for Cuvier function determined form. Geoffroy held that Nature formed nothing new, but adapted existing "materials of organisation" to meet new needs. Cuvier, on the other hand, was always ready to admit Nature's power to form entirely new organs in response to new functional requirements.

The evolutionists followed Geoffroy rather than Cuvier. They laid great store by homological resemblances, and dismissed analogies of structure as of little interest. They were singularly unwilling to admit the existence of convergence or of parallel evolution, and they held very firmly the distinctively Geoffroyan view that Nature is so limited by the unity of composition that she can and does form no new organs.

By no one has this underlying principle of evolutionary morphology been more explicitly recognised than by Hubrecht, who in his paper of 1887, after summarising the points of resemblance between Nemertines and Vertebrates which led him to assume a genetic connection between them, writes as follows:—"At the base of all the speculations contained in this chapter lies the conviction, so strongly insisted upon by Darwin, that new combinations or organs do not appear by the action of natural selection unless others have preceded, from which they are gradually derived by a slow change and differentiation.

"That a notochord should develop out of the archenteric wall because a supporting axis would be beneficial to the animal may be a teleological assumption, but it is at the same time an evolutional heresy. It would never be fruitful to try to connect the different variations offered, e.g., by the nervous system throughout the animal kingdom, if similar assumptions were admitted, for there would be then quite as much to say for a repeated and independent origin of central nervous systems out of indifferent epiblast just as required in each special case. These would be steps that might bring us back a good way towards the doctrine of independent creations. The remembrance of Darwin's, Huxley's, and Gegenbaur's classical foundations, and of Balfour's and Weismann's brilliant superstructures, ought to warn us away from these dangerous regions" (p. 644).

This same prejudice lies at the root of the idea of Functionswechsel, in spite of the general functional orientation of that idea.

Dohrn's constant assumption is that Nature makes shift with old organs wherever possible, instead of forming new ones. He derives gill-slits from segmental organs, fins and limbs from gills, ribs from gill-arches, and so on, instead of admitting that these organs might quite as well have arisen independently. He objects on principle to the origin of organs de novo. Thus, rebutting the suggestion that certain organs which are not found in the lower Vertebrates might have arisen as new formations, he writes:—"Against this supposition the whole weight of all those objections can be directed that are to be brought in general against the method of explanation which consists in appealing without imperative necessity to the Deus ex machina, 'New formation,' which is neither better nor worse than Generatio equivoca" (p. 21).

Of a similar nature was the objection to convergence.[456]

Why, we may ask, were morphologists so unwilling to admit the creative power of life? Dohrn, for instance, was fully aware of the great transforming influence exerted by function upon form—his theory of Functionswechsel regards as the most powerful agent of change the activity of the animal, its effort to make the best use of its organs, to apply them at need in new ways to meet new demands. Why then did he not go a step further and admit that the animal could by its own subconscious efforts form entirely new organs? Why did most morphologists join with him in belittling the organism's power of self-transformation?

The reasons seem to have been several. There is first the fundamental reason, that the idea of an active creative organism is repugnant to the intelligence, and that we try by all means in our power to substitute for this some other conception. In so doing we instinctively fasten upon the relatively less living side of organisms—their routine habits and reflexes, their routine structure—and ignore the essential activity which they manifest both in behaviour and in form-change.

We tend also to lay the causes of form-change, of evolution, as far as possible outside the living organism. With Darwin we seek the transforming factors in the environment rather than within the organism itself. We fight shy of the Lamarckian conception that the living thing obscurely works out its own salvation by blind and instinctive effort. We like to think of organisms as machines, as passive inventions[457] gradually perfected from generation to generation by some external agency, by environment or by natural selection, or what you will. All this makes us chary of believing that Nature is prodigal of new organs.

Other causes of the unwillingness of morphologists to admit the new formation of organs are to be sought in the main principle of pure morphology itself, that the unity of plan imposes an iron limit upon adaptation, and in the powerful influence exercised at the time by materialistic habits of thought. Teleology had become a bugbear to the vast majority of biologists, and all real understanding of the Cuvierian attitude seems, in most cases, to have been lost, although, curiously enough, teleological conceptions were often unconsciously introduced in the course of discussions on the "utility" of organs in the struggle for existence.

Evolutionary morphology, being for the most part a form of pure or non-functional morphology, agreed then in all essential respects with pre-evolutionary or transcendental morphology.

But it contained the germ of a new conception which threw a new light upon the whole science of morphology. This was the conception of the organism as an historical being.

We have seen this thought expressed with the utmost clearness by Darwin himself (supra, p. 233). In his eyes the structure and activities of the living thing were a heritage from a remote past, the organism was a living record of the achievements of its whole ancestral line. What a light this conception threw upon all biology! "When we no longer look at an organic being as a savage looks at a ship as something wholly beyond his comprehension; when we regard every production of Nature as one which has had a long history; when we contemplate every complex structure and instinct as the summing-up of many contrivances, each useful to the possessor, in the same way as any great mechanical invention is the summing-up of the labour, the experience, the reason, and even the blunders of numerous workmen; when we thus view each organic being, how far more interesting—I speak from experience—does the study of natural history become!" (Origin, 6th ed., pp. 665-6).

Sedgwick expressed the same thing from the morphological point of view when he wrote, with reference to the ancestral significance of the blastopore:—"If there is anything in the theory of evolution, every change in the embryo must have had a counterpart in the history of the race, and it is our business as morphologists to find it out" (p. 49, 1884).

By the evolution-theory the problems of form were linked indissolubly with the problem of heredity. Unity of plan could no longer be explained idealistically as the manifestation of Divine archetypal ideas; it had a real historical basis, and was due to inheritance from a common ancestor. The evolution-theory gave meaning and intelligibility to the transcendental conception of the unity of plan; in particular it supplied a simple and satisfying explanation of those puzzling vestigial organs, whose existence was such a stumbling-block to the teleologists. It enabled the biogenetic law to be substituted for the laws of Meckel-Serres and von Baer, as being in some measure a combination and interpretation of both.

Where the concept of evolution proved itself particularly useful was in the interpretation of structures which were not immediately conditioned by adaptation to present requirements, such as, for instance, the arrangement of gill-slits and aortic arches in the fœtus of land Vertebrates. Such "heritage characters" could only be explained on the hypothesis that they had once had functional or adaptational meaning. Why, for instance, should the blastopore so often appear as a long slit, closing by concrescence, unless this had been the original method of its formation in remote Cœlenterate ancestors?

The point hardly requires elaboration, since it has become an integral part of all our thinking on biological problems. It may be as well, however, for the sake of continuity, to give one or two examples of the historical interpretation of animal structures. The first may conveniently be the phylogenetic interpretation of the contrast between "membrane" and "cartilage" bones.

In his Grundzüge of 1870, Gegenbaur made the suggestion that the investing or membrane bones were derived phylogenetically from integumentary ossifications, and this was worked out in detail a few years later by O. Hertwig.[458]

Many years before, several observers—J. Müller, Williamson, and Steenstrup—had been struck with the resemblance existing between the placoid scales and the teeth of Elasmobranch fishes. Hertwig followed up this clue, and came to the conclusion not only that placoid scales and teeth were strictly homologous, but also that all membrane bones were derived phylogenetically from ossifications present in the skin or in the mucous membrane of the mouth, just as cartilage bones were derived from the cartilaginous skeletons of the primitive Vertebrates. In some cases this manner of derivation could even be observed in ontogeny, as Reichert had seen in the Newt, where certain bones in the roof of the mouth are actually formed by the concrescence of little teeth, (supra, p. 163). Hertwig considered that the following bones were originally formed by coalescence of teeth—parasphenoid, vomer, palatine, pterygoid, the tooth-bearing part of the pre-maxillary, the maxillary, the dentary and certain bones of the hyo-mandibular skeleton of Teleosts. All the investing bones (Deckknochen) of the skull were of common origin, and could be traced back to integumentary skeletal plates, which in the ancestral fish formed a dense carapace.

These conclusions were accepted by Kölliker himself, who wrote in his Entwickelungsgeschichte (1879)—"The distinction between the primary or primordial, and the investing or secondary bones is from the morphological standpoint sharp and definite. The former are ossifications of the (cartilaginous) primordial skeleton, the latter are formed outside this skeleton, and are probably all ossifications of the skin or the mucous membrane" (p. 464).

Gegenbaur[459] consistently upheld the phylogenetic derivation of investing bones from dermal ossifications, and even went further and derived substitutionary bones as well from the integument, thus establishing a direct comparison between the skeletal formations of Vertebrates and Invertebrates. Investing bones were actual integumentary ossifications which had gradually sunk beneath the skin to become part of the internal skeleton; substitutionary bones were produced by cells (osteoblasts) which were ultimately derived from the integument.[460]

A further instance of the historical interpretation of animal structure, taken from quite a different field, is afforded by the speculations of Dollo[461] on the ancestral history of the Marsupials. In a brilliant paper of 1880[462] Huxley made the suggestion that the ancestors of Marsupials were arboreal forms. "I think it probable," he wrote, "from the character of the pes, that the primitive forms, whence the existing Marsupialia have been derived, were arboreal animals; and it is not difficult, I conceive, to see that, with such habits, it may have been highly advantageous to an animal to get rid of its young from the interior of its body at as early a period of development as possible, and to supply it with nourishment during the later periods through the lacteal glands, rather than through an imperfect form of placenta" (p. 655). Dollo followed up this suggestion, which had in the meantime been strengthened by Hill's discovery of a true allantoic placenta in Perameles, by demonstrating in the foot of present-day Marsupials certain features which could only be interpreted as inherited from a time when the ancestors of Marsupials were tree-living animals. These were the occurrence of an opposable big toe (when this was present at all), the great development of the fourth toe, the reduction and partial syndactylism of the second and third toes, and in some cases the regression of the nails. These characters were shown to be typical of arboreal Vertebrates, and their occurrence in forms not arboreal indicated that these were descended from tree-living ancestors. Traces of an arboreal ancestry could be demonstrated even in the marsupial mole Notoryctes.

These are only two examples out of hundreds that might be given. Present day structure was interpreted in the light of past history; the common element in organic form was seen to be due to common descent; the existence of vestigial and non-functional organs was no longer a riddle.

There was even a tendency to concentrate attention upon the historical side of structure, upon what the animal passively inherited rather than upon what it personally achieved. Homologies were considered more interesting than analogies, vestigial organs more interesting than fœtal and larval adaptations. Convergence was anathema. The dead-weight of the past was appreciated at its full and more than its full value; and the essential vital activity of the living thing, so clearly shown in development and regeneration, was ignored or forgotten.

But evolutionary morphology for all practical purposes was a development of pure or idealistic morphology, and was powerless to bring to fruit the new conception with which evolution-theory had enriched it. The reason is not far to seek. Pure morphology is essentially a science of comparison which seeks to disentangle the unity hidden beneath the diversity of organic form. It is not immediately concerned with the causes of organic diversity—that is rather the task of the sciences of the individual, heredity and development. To take an example—the recapitulation theory may legitimately be used as a law of pure morphology, as stating the abstract relation of ontogeny to phylogeny, and the probable line of descent of any organism may be deduced from it, as a mere matter of the ideal derivation of one form from another; but an explanation of the reason for the recapitulation of ancestral history during development can clearly not be given by pure morphology unaided. From the fact that the common starfish shows in the course of its development distinct traces of a stalk[463] it is possible to infer, taking other evidence also into consideration, that the ancestors of the starfish were at one stage of their existence stalked and sessile organisms. But this leaves unanswered the question as to how and why the starfish does still repeat after so many millions of years part of the organisation of one of its remote ancestors. Why is this feature retained, and by what means has it been conserved through countless generations? It is clear that the answer can be given only by a science of the causes of the production and retention of form, by a causal morphology, based upon a study of heredity and development.

From the point of view of the pure morphologist the recapitulation theory is an instrument of research enabling him to reconstruct probable lines of descent; from the standpoint of the student of development and heredity the fact of recapitulation is a difficult problem whose solution would perhaps give the key to a true understanding of the real nature of heredity.

To make full use of the conception of the organism as an historical being it is necessary then to understand the causal nexus between ontogeny and phylogeny.

We shall see in the next chapter that the transformation of morphology from a comparative to a causal science did take place towards the end of the century, and that some progress was made towards an understanding of the relation between individual development and ancestral history, particularly by Roux and Samuel Butler, working with the fruitful Lamarckian conception of the transforming power of function.

CHAPTER XVIII

THE BEGINNINGS OF CAUSAL MORPHOLOGY

Until well into the 'eighties animal morphology remained a purely descriptive science, content to state and summarise the relations between the coexistent and successive form-states of the same and of different animals. No serious attempt had been made to discover the causes which led to the production of form in the individual and in the race.

It is true that evolution-theory had offered a simple solution of the great problem of the unity in diversity of animal forms, but this solution was formal merely, and went little beyond that abstract deduction of more complex from simpler forms, which had been the main operation of pre-evolutionary morphology. Little was known of the actual causes of ontogeny, and nothing at all of the causes of phylogeny; it was, for instance, mere rhetoric on Haeckel's part to proclaim that phylogeny was the mechanical cause of ontogeny.

Animal physiology, on its side, had developed in complete isolation from morphology into a science of the functioning of the adult and finished animal, considered as a more or less stable physico-chemical mechanism. Since the days of Ludwig, Claude Bernard and E. du Bois Reymond, the physiologists' chief care had been to analyse vital activities into their component physical and chemical processes, and to trace out the interchange of matter and energy between the organism and its environment. Physiologists had left untouched, perhaps wisely, the much more difficult problem of the causes of the development of form. For all practical purposes they took the animal-machine as given, and did not trouble about its mode of origin. They held indeed that form-production was due to a complex of physico-chemical causes, which they hoped some day to unravel;[464] but this future physiology of development remained quite embryonic.

Physiology then had not really come into contact with the problems of form, and it could give the morphologist no direct help when he turned to investigate the causes of form-production. It had, however, a determining influence upon the methods of those who first broke ground in this No Man's Land between morphology proper and physiology. But it is significant that it was a morphologist and not a physiologist that did the first spade-work.

The pioneer in this field, both as investigator and as thinker, was W. Roux, who sketched in the 'eighties the main outlines of a new science of causal morphology, to which he gave the name of Entwicklungsmechanik. The choice of name was deliberate, and the word implied, first, that the new science was essentially an investigation of the development of form, not of the mode of action of a formed mechanism, and second, that the methods to be adopted were mechanistic.[465]

Though Roux was the only begetter of the science of Entwicklungsmechanik, he was, of course, not the first to investigate experimentally the formative processes of animal life. Study of regeneration dates back to Trembley (1740-44), Réaumur (1742), Bonnet (1745), and Spallanzani (1768-82),[466] and in the years preceding Roux's activity good work was done by Philipeaux. A beginning had been made with experimental teratology by E. Geoffroy St Hilaire and others, and the work of C. Dareste[467] remains classical. Back in the 18th century, some of John Hunter's experiments had a bearing upon the problems of form; his work on transplantation was followed up in the 19th century by Flourens, p. Bert, Ollier and many others. In founding in 1872 the Archives de Zoologie expérimentale et générale H. de Lacaze-Duthiers put forward in his introduction a powerful plea for the use of the experimental method in zoology.

In some ways more directly connected with Entwicklungsmechanik was His's attempt in 1874[468] to explain on mechanical principles the formation of certain of the embryonic organs by the bendings and foldings of tubes or plates of cells. "His compared the various layers of the chick embryo to elastic plates and tubes; out of these he suggested that some of the principal organs might be moulded by mere local inequalities of growth—the ventricles of the brain, for instance, the alimentary canal, the heart—and he further succeeded in imitating the formation of these organs by folding, pinching, and cutting india-rubber tubes and plates in various ways."[469]

But Roux was undoubtedly the first to make a systematic survey of the problems to be solved and to work out an organised method of attack. His earliest work deals with the important problem of functional adaptation—its importance to the organism, and its possible mechanistic explanation. The first paper[470] was a study of the branching and distribution of the arteries in the human body (1878), and a second paper on the same subject followed in 1879.[471]

In these papers Roux showed how the development of the blood-vascular system was largely determined by direct adaptation to functional requirements, and he inferred the existence in the vascular tissues of certain vital properties, in virtue of which the functional adaptation of the blood-vessels came about. Thus the intima or inner lining must possess the faculty of so reacting to the friction set up by the blood-current as to oppose the least possible resistance to its flow; the muscular coats must react to increased pressure by growing thicker, and so on.

These papers were followed in 1881 by his well-known book, Der Kampf der Theile im Organismus, which contained the working-out of his mechanistic explanation of functional adaptation, and most of the elements of his general "causal-analytical" theory of form production. The significance of the book was popularly considered at the time to lie in its supposed application of the selection idea to the explanation of the internal adaptedness of animal structure—in the theory of "cellular selection," and the book owed its success to its fitting in so well with the prevalent Darwinism of the day. But its real importance, as a big step towards causal morphology, was naturally not so fully appreciated.

During the next few years Roux continued his studies on functional adaptation,[472] and at the same time made a new departure by inaugurating, almost contemporaneously with the physiologist Pflüger, the study of experimental embryology. Isolated observations had previously been made upon the development of single blastomeres or parts of blastulæ, by Haeckel and Chun for instance,[473] but Roux[474] and Pflüger[475] were the first to investigate the subject systematically, choosing for their work the egg of the frog.[476] Roux continued for many years to follow up this line of work.[477]

In 1890 he drew up a programme and manifesto[478] of Entwicklungsmechanik as "an anatomical science of the future," and in 1895 he founded the famous Archiv für Entwicklungsmechanik,[479] publishing in the same year the two large volumes of his collected papers,[480] of which the first volume dealt with functional adaptation, the second with experimental embryology.

His subsequent work includes several important general papers;[481] besides a number of special memoirs dealing with the factors of development, and with his original subject, functional adaptation.[482]

In our sketch of his views we shall have occasion to refer particularly to his publications of 1881, 1895 (the Einleitung), 1902, 1905, and 1910.

Although Roux's biological philosophy is out-and-out mechanistic, he yet recognises the difficulty, even the impossibility, of straightway reducing development to the physico-chemical level. He tries to steer a course midway between the simplicist conceptions of the materialists and the "metaphysics" of the neo-vitalist school, which the experimental study of development and regeneration soon brought into being. In 1895 he writes:—"The too simple mechanistic conception on the one hand, and the metaphysical conception on the other represent the Scylla and Charybdis, between which to sail is indeed difficult, and so far by few satisfactorily accomplished; it cannot be denied that with the increase of knowledge the seduction of the second has lately notably increased" (p. 23).

The via media adopted by Roux is the analysis of development, not directly into simple physico-chemical processes, but into more complex organic processes dependent upon the fundamental properties of living matter. The aim of Entwicklungsmechanik is defined by Roux to be the reduction of developmental events to the fewest and simplest Wirkungsweisen, or causal processes.[483] Two classes of causal processes may be distinguished, as "complex components" and "simple components" of development. The latter are directly explicable by the laws of physics and chemistry; the former, while in essence physico-chemical, are yet so very complicated that they cannot at present be reduced to physico-chemical terms. The ultimate aim of Entwicklungsmechanik is to reduce development to its "simple components," but its main task at the present day and for many years to come is the analysis of development into its "complex components."

These complex components must be accepted as having much of the validity of physical and chemical laws. They are mysterious in the sense that they cannot yet be explained mechanistically, but they are constant in their action, and under the same conditions produce always the same effect—hence they may be made the subject of strictly scientific study. They represent biological generalisations, in their way of equal validity with the generalisations of physics and chemistry.

The principal "complex components" which Roux recognises are somewhat as follows:—First come the elementary cell-functions of assimilation and dissimilation, growth, reproduction and heredity, movement and self-division (as a special co-ordination of cell-movements). Then at a somewhat higher level, self-differentiation, and the trophic reaction to functional stimuli. Components of even greater complexity may also be distinguished, as, for instance, the biogenetic law. The various tropisms exhibited in development may be regarded as "directive" complex components. There must be added, not as being itself a component, but rather as a mode or peculiar property of all functioning, the omnipresent faculty of self-regulation.

It will be noticed that Roux's "complex components" are simply the general properties or functions of organised matter.

Expressing Roux's thought in another way, we might say that life can only be defined functionally, i.e., by an enumeration of the "complex components" or elementary functions which all living beings manifest, even down to the very simplest. "Living beings," writes Roux, "can at present be defined with any approach to completeness only functionally, that is to say, through characterisation of their activities, for we have an adequate acquaintance with their functions in a general way, though our knowledge of particulars is by no means complete" (p. 105, 1905). Defined in the most general and abstract way, living things are material objects which persist in spite of their metabolism, and, by reason of their power of self-regulation, in spite also of the changes of the environment. This is the "functional minimum-definition of life" (pp. 106-7, 1905).

We may now go on to consider the relation of function to form throughout the course of development. Roux distinguishes in all development two periods, in the first of which the organ is formed prior to and independent of its function, while in the second the differentiation and growth of the organ are dependent on its functioning. Latterly (1906 and 1910) Roux has distinguished three periods, counting as the second the transition period when form is partly self-determined, partly determined by functioning. As this conception of Roux's is of the greatest importance we shall follow it out in some detail.

The idea was first elaborated in the Kampf der Theile (1881), where he wrote:—"There must be distinguished in the life of all the parts two periods, an embryonic in the broad sense, during which the parts develop, differentiate and grow of themselves, and a period of completer development, during which growth, and in many cases also the balance of assimilation over dissimilation, can come about only under the influence of stimuli" (p. 180). There is thus a period of self-differentiation in which the organs are roughly formed in anticipation of functioning, and a period of functional development in which the organs are perfected through functioning and only through functioning. The two periods cannot be sharply separated from one another, nor does the transition from the one to the other occur at the same time in the different tissues and organs.

The conception is more fully expressed in 1905 as follows:—"This separation (of development into two periods) is intended only as a first beginning. The first period I called the embryonic period κατ' ἐξοχήν, or the period of organ-rudiments. It includes the 'directly inherited' structures, i.e., the structures which are directly predetermined in the structure of the germ-plasm, as, for instance, the first differentiation of the germ, segmentation, the formation of the germ-layers and the organ-rudiments, as well as the next stage of 'further differentiation,' and of independent growth and maintenance, that is, of growth and maintenance which take place without the functioning of the organs.

"This is accordingly the period of direct fashioning through the activity of the formative mechanism implicit in the germ-plasm, also the period of the self-conservation of the formed parts without active functioning.

"The second period is the period of 'functional form-development.' It includes the further differentiation and the maintenance in their typical form of the organs laid down in the first period; and this is brought about by the exercise of the specific functions of the organs. This period adds the finishing touches to the finer functional differentiation of the organs, and so brings to pass the 'finer functional harmony' of all organs with the whole. The formative activity displayed during this period depends upon the circumstance that the functional stimulus, or rather the exercise by the organs of their specific functions, is accompanied by a subsidiary formative activity, which acts partly by producing new form and partly by maintaining that which is already formed.... Between the two periods lies presumably a transition period, an intermediary stage of varying duration in the different organs, in which both classes of causes are concerned in the further building-up of the already formed, those of the first period in gradually decreasing measure, those of the second in an increasing degree" (pp. 94-6, 1905).

In the first period the organ forms or determines the function, in the second period the function forms the organ, or at least completes its differentiation. It is characteristic that in the first period functionally adapted structure appears in the complete absence of the functional stimulus.

The explanation of the difference between the two periods is to be found in the different evolutionary history of the characters formed during each. First-period characters are inherited characters, and taken together constitute the historical basis of the organism's form and activity; second-period characters are those of later acquirement which have not yet become incorporated in the racial heritage.

Inherited characters appear in development in the absence of the stimulus that originally called them forth; acquired characters are those that have not yet freed themselves from this dependence upon the functional stimulus. First-period characters were originally, like second-period characters, entirely dependent for their development upon the functional stimuli in response to which they arose, and only gradually in the course of generations did they gain that independence of the functional stimulus which stamps them as true inherited characters. Speaking of the formative stimuli which are active in second-period development, Roux writes:—"These stimuli can also produce new structure, which if it is constantly formed throughout many generations finally becomes hereditary, i.e., develops in the descendants in the absence of the stimuli, becomes in our sense embryonic" (p. 180, 1881). Again, "form-characteristics which were originally acquired in post-embryonic life through functional adaptation may be developed in the embryo without the functional stimulus, and may in later development become more or less completely differentiated, and retain this differentiation without functional activity or with a minimum of it. But in the continued absence of functional activity they become atrophied ... and in the end disappear" (p. 201, 1881).

This conception of the nature of hereditary transmission is an important one, and constitutes the first big step towards a real understanding of the historical element in organic form and activity. It supplies a practical criterion for the distinguishing of "heritage" characters from acquired characters, of palingenetic from cenogenetic—a criterion which descriptive morphology was unable to find.[484] The introduction of a functional moment into the concept of heredity was a methodological advance of the first importance, for it linked up in an understandable way the problems of embryology, and indirectly of all morphology, with the problem of hereditary transmission, and gave form and substance to the conception of the organism as an historical being.

It is this element in Roux's theories that puts them so far in advance of those of Weismann. Weismann did not really tackle the big problem of the relation of form to function, and he left no place in his mechanical system of preformation for functional or second-period development; he conceived all development to be in Roux's sense embryonic, and due to the automatic unpacking of a complex germinal organisation. Roux himself was to a certain extent a preformationist, for the development of his first-period characters is conditioned by the inherited organisation of the germ-plasm, and is purely automatic. It was indeed his experiments on the frog's egg (1888) that supplied some of the strongest evidence in favour of the mosaic theory of development. The number of Anlagen which he postulates in the germ is however small, and the germ-plasm in his conception of it has a relatively simple structure (p. 103, 1905).

The transmission of acquired characters forms, of course, an integral part of Roux's conception of heredity and development, for without this transmission second-stage characters could not be transformed into first-stage characters. He discusses this difficult question at some length in the Kampf der Theile, coming to the conclusion that such transmission takes place in small degree and gradually, and that many generations are required before a new character can become hereditary. He thinks that acquired characters are probably transmitted at the chemical level. It is conceivable that acquired form-changes are dependent on chemical changes, or are correlative with such, and that, since the germ-cells stand in close metabolic relations with the soma, these chemical changes may soak through to the germ-cells and so modify them that a predisposition will appear in the descendants towards similar form-changes.[485] From this point of view the problem of transmission might be merged in the broader problem of the production of form through chemical processes—the central problem of all development.

Inherited characters develop by an automatic process of self-differentiation, and the separate parts of the embryo show during this first period a surprising functional independence of one another. But this state of things changes progressively as the second period is reached, until finally all form-production and maintenance and all correlation depend upon functioning. It is in the first period of automatic development through internal "determining" factors that the "developmental" functions in the strict sense, e.g. automatic growth, division and self-differentiation, are most clearly shown. In the second or "functional" period the formative influence of function upon structure comes into play, and development becomes largely a matter of "functional adaptation" to functional requirements.

All structure, according to Roux, is either functional or non-functional. The former includes all structure that is adapted to subserve some function. "Such 'functional structures' are, for example, the composition of striated muscle fibres out of fibrillæ and these out of muscle-prisms, or again the length and thickness of the muscles, the static structure of the bones, the composition of the stomach and the blood-vessels out of longitudinal and circular fibres, the external shape of the vertebral centra and of the cuneiform bones of the foot" (p. 73, 1910). Indeed, as Cuvier had already pointed out, practically every organ in the body shows a functional structure which is accurately and minutely adjusted to the function it is intended to perform. Thus, to take some further examples, the arteries are admirably adapted as regards size of lumen, elasticity of wall, direction of branching, to conduct the blood to all parts of the body with the least possible waste of the propelling power through frictional resistance. So, too, the spongy substance of the long bones is arranged in lamellæ which take the direction of the principal stresses and strains which fall upon the bones in action.

Functional structure may be formed either in the first or in the second period of development, may be either inherited or acquired, but it reaches its full differentiation only in the second period, i.e., under the influence of functioning. Practically speaking, functional structure is directly dependent for its full development and for its continued conservation upon the exercise of the particular function which it serves. In the second period, but not in the first, increased use leads to hypertrophy of the functional structure, disuse to atrophy.

From functional structure is to be distinguished nonfunctional structure, which has no relation to the bodily functions—is neither adapted to perform any of these, nor has arisen as a by-product of functional activity. "To this category belong, for example, among typical structures, the triangular form of the cross-section of the tibia, the dolicocephalic or brachycephalic shape of the skull, most of the external characters distinguishing genera and species, many of the external features of the embryo which change in the course of development, besides most of the abnormal forms shown by monstrosities, tumours, etc." (p. 74, 1910). Non-functional structure is not affected by functional adaptation, and may accordingly be left out of consideration here.

Now the influence of functioning upon the form and structure of an organ is twofold. There is first the immediate change brought about by the very act of functioning—for example, the shortening and thickening of skeletal muscles when they act. This is a purely temporary change, for the organ at once returns to its normal quiescent state as soon as it ceases to function. Such temporary functional change, brought about in the moment of functioning, is usually dependent for its initiation upon some neuro-muscular mechanism, though it may be elicited also by a chemical stimulus. It is thus always a phenomenon of "behaviour." "From such temporary changes are sharply to be distinguished all permanent alterations which first appear in perceptible fashion through oft-repeated or long-continued, enhanced functional activity. These produce a new and lasting internal equilibrium of the organ, consisting in an insertion of new molecules or a rearrangement of old. For this reason they outlast the periods of functional form-change, or, if as in the case of the muscles they themselves alter during functional activity, they regain their state when the organ ceases to function" (p. 72, 1910). "Oft-repeated exercise or heightened exercise of the specific functions, or repeated action of the functional stimuli which determine them, produces, as we have said before, true form-changes as a by-product. These are of two kinds. In so far as these form-changes facilitate the repetition of the specific functions, I have called them functional adaptations.... Such as do not improve the functioning of the organ are indeed by-products of functioning, but without adaptive character; they do not belong to the class of functional adaptations at all" (p. 75, 1910).

We may now enquire in what way functional adaptations can arise as by-products of functioning.

It is clear that natural selection in the sense of individual or "personal" selection cannot adequately explain the origin of functional structure and the functional harmony of structure, for thousands of cells would have to vary together in a purposive way before any real advantage could be gained in the struggle for existence, and it is in the highest degree unlikely that this should come about by chance variation.[486] The development of purposive internal structure is only to be explained by the properties of the tissues concerned.

In illustration and proof of the statement that functional adaptation is due to the properties of the tissues we may adduce the development and regulation of the blood-vascular system, which has been thoroughly studied from this point of view by Roux and Oppel (1910).

It appears that only the very first rudiments of the vascular system are laid down in the short first period of automatic non-functional development. All the subsequent growth and differentiation of the blood-vessels falls into the second period, and is due wholly or in great part to direct functional adaptation to the requirements of the tissues. Thus from the rudiments formed in the first period there sprout out the definitive vessels in direct adaptation to the food-consumption of the tissues they are to supply. The size, direction and intimate structure of these vessels are accurately adjusted to the part they play in the economy of the whole, and this adjustment is brought about in virtue of the peculiar properties or reaction-capabilities of the different tissues of which the blood-vessels are composed.

The properties which Roux finds himself compelled to postulate in the vascular tissues, after a thorough-going analysis of the different kinds of functional adaptation shown by the blood-vessels, are summarised by him as follows:—

"(1) The faculty—depending on a direct sensibility possessed by the endothelium and perhaps also by the other layers of the intima—of yielding to the impact of the blood, so far as the external relations of the vessel permit. In this way the wall adapts itself to the hæmodynamically conditioned 'natural' shape of the blood-stream, and reaches this shape as nearly as possible." Through this faculty of the lining tissue of the blood-vessels, the size of the lumen and the direction of branching are so regulated as to oppose the least possible resistance to the flow of the blood.

"(2) The faculty possessed by the endothelium of the capillaries of each organ of adapting itself qualitatively to the particular metabolism of the organ." This adaptedness of the capillaries is, however, more usually an inherited state, i.e., brought about in the first period of development.

"(3) The faculty possessed by the capillary walls of being stimulated to sprout out and branch by increased functioning, i.e., by increased diffusion, and their power to exhibit a chemically conditioned cytotropism, which causes the sprouts to find one another and unite. A similar process can be directly observed in isolated segmentation-cells, which tend to unite in consequence of a power of mutual attraction.

"(4) The faculty of developing normal arterial walls in response to strong intermittent pressure, and normal venous walls in response to continuous lesser pressure." It has been shown, for instance, by Fischer and Schmieden that in dogs a section of vein transplanted into an artery takes on an arterial structure, at least as regards the circular musculature, which doubles in thickness.

"(5) The power to regulate the normal[487] length of the arteries and veins, in adaptation to the growth of the surrounding tissues, in such a way that the stretching action of the blood-stream brings the vessel to its proper functional length.

"(6) The power to form, in response to slight increases in longitudinal tension, new structural parts which take their place alongside the existing longitudinal fibres.

"(7) The power to regulate the width of the circular musculature according to the degree of food-consumption by the tissues, in response to nerve impulses initiated in these tissues.

"(8) The power possessed by the circular musculature of responding to such continuous functional widening, by the formation of new structural parts in the circular musculature, and so of widening the vessel permanently or by this new formation of muscular fibres thickening the circular musculature.

"(9) The faculty of being stimulated by increased blood-pressure to produce the same structural changes as mentioned in par. 8, though here the response is otherwise conditioned" (pp. 126-7, 1910).

It is by virtue of the tissue-properties detailed above that the complex functional adaptations of the blood-vessels come about.

The development of the vascular system is no mere automatic and mechanical production of form, apart from and independent of functioning; it implies a living and co-ordinated activity of the tissues and organs concerned, a power of active response to foreseen and unforeseen contingencies. Form is then not something fixed and congealed—it is the ever-changing manifestation of functional activity. "Since most of the structure and form of the blood-vessels arises in direct adaptation to function, the vessels of adult men and animals are no fixed structures, which, once formed, retain their form and structural build unchanged throughout life; on the contrary, they require even for their continued existence the stimulus of functional activity.... The fully formed blood-vessels are no static structures, such as they appear to be according to the teaching of normal histology, and such as they have long been taken to be. Observation and description of normal development never shows us anything but the visible side of organic happenings, the products of activity, and leaves us ignorant of the real processes of form-development and form-conservation, and of their causes" (p. 125, 1910).

The real thing in organisation is not form but activity. It is in this return to the Cuvierian or functional attitude to the problems of form that we hold Roux's greatest service to biology to consist. The attitude, however, seems to smack of vitalism, and Roux, as we have seen, is no vitalist. He holds that the marvellous and apparently purposive tissue-qualities which underlie all processes of functional adaptation have arisen "naturally," in the course of evolution, by the action of natural selection upon the various properties, useful and useless, which appeared fortuitously in the primary living organisms. He is, moreover, deeply imbued with the materialistic philosophy of his youth, and it is indeed one of the chief characteristics of his system that he states the fundamental properties or qualities of life in terms of metabolism. A vital quality is for Roux a special process or mode of assimilation. The faculty of "morphological assimilation" whereby form is imposed upon formless chemical processes is the ultimate term of Roux's analysis—"the most general, most essential, and most characteristic formative activity of life" (p. 631, 1902).

We have now to consider very briefly the early results achieved by Roux's fellow-workers in the field of causal morphology. As D. Barfurth points out,[488] the years 1880-90 saw a general awakening of interest in experimental morphology, and it is hard to say whether Roux's work was cause or consequence. "There fall into this period," writes Barfurth, "the experimental investigations by Born and Pflüger on the sexual difference in frogs (1881), by Pflüger on the parthenogenetic segmentation of Amphibian ova, on crossing among the Amphibia, and on other important subjects (1882). In the following year (1883) appeared two papers of fundamental importance, by E. Pflüger and W. Roux: Pflüger publishing his researches on 'the influence of gravity on cell-division,' Roux his experimental investigations on 'the time of the determination of the chief planes in the frog-embryo.'... In the same year appeared A. Rauber's experimental studies 'on the influence of temperature, atmospheric pressure, and various substances on the development of animal ova,' which have brought many similar works in their train. The following year (1884) saw a lively controversy on Pflüger's gravity-experiments with animal eggs, in which took part Pflüger, Born, Roux, O. Hertwig and others, and in this year appeared work by Roux dealing with the experimental study of development, and in particular giving the results of the first definitely localised pricking-experiments on the frog's egg (in the Schles. Gesell. f. vaterl. Kultur, 15th Feb. 1884), also the important researches of M. Nussbaum and Gruber (followed up later by Verworn, Hofer and Balbiani) on Protozoa, and other experimental work" (pp. xi.-xii.).

In 1888 appeared a famous paper by W. Roux,[489] in which he described how he had succeeded in killing by means of a hot needle one of the two first blastomeres of the frog's egg, and how a half-embryo had developed from the uninjured cell. Some years before[490] he had enunciated, at about the same time as Weismann, the view that development was brought about by a qualitative division of the germ-plasm contained in the nucleus, and that the complicated process of karyokinetic or mitotic division of the nucleus was essentially adapted to this end. He conceived that development proceeded by a mosaic-like distribution of potencies to the segmentation-cells, that, for instance, the first segmentation furrow separated off the material and potencies for the right half of the embryo from those for the left half. He had tried to show experimentally that the first furrow in the frog's egg coincided with the sagittal plane of the embryo,[491] and his later success in obtaining a half-embryo from one of the first two blastomeres seemed to establish the "mosaic theory" conclusively.