There has recently come before me a fact which has a significant bearing on the hypothesis of Constitutional units: serving, indeed, to give an apparently conclusive proof of its truth. Before stating it, however, I may with advantage re-state the several evidences already assigned in support of it.

1. First comes the à priori reason. These units in the germ of an organism which cause development into a special structure, cannot be chemical units—cannot be simply molecules of proteid substance in one or other of its forms; since these are not special to any type of creature but common to all creatures. Nor can they be what we may call morphological units—the cells or protoplasts; because in the early stages of development the cells of one organism are indistinguishable from those of others, and because were cells the units of composition there could be no interpretation of what are called unicellular organisms—nothing to account for the innumerable varieties of them. Hence, of necessity, the structural elements of which each organism is built, being neither proteid molecules nor cells, must be something between them: probably some complex combination of different isomeric forms of proteids.

2. That units of such natures are the essential components of each species of organism, is shown by the fact that in low types of creatures, little differentiated into special tissues, any considerable portion of the body will, when separated, begin to assume the structure proper to the species—a truth recently shown afresh by Prof. T. H. Morgan’s experiments on the regeneration of Planaria maculata (already referred to in § 206) showing that various fragments cut out develop into new individuals, and that when, being too small they die before doing this, there is always an abortive attempt to assume the specific structure.

3. This truth that a portion of undifferentiated tissue, if adequate in quantity, assumes the structure of the type, illustrating as it does the proclivity of the constitutional units towards the structure of the species, allies itself with the phenomena of both agamogenesis and gamogenesis. The first of these shows us how a fissiparously-detached portion of the parental tissue takes on the same form as the parent; and the second shows how those small detached portions distinguished as sperm-cell and germ-cell also, when united and supplied with the needful materials, do the same thing.

4. But the set of phenomena following the union of sperm-cell and germ-cell differ in a certain way from those which follow when a gemma or other unfertilized portion of parental tissue is detached. The incomprehensibleness of this difference as otherwise contemplated, and the partial comprehensibleness of it when joined with the hypothesis of physiological units, furnish a further support for the hypothesis.

The familiar truth learnt by the tyro in algebra that an apparent solution which contains the unknown quantity is no solution, is a truth apt to be overlooked in other spheres than the algebraic. An illustration is supplied by the answer once given in Parliament to the question “What is an Archdeacon?”—“One who discharges archidiaconal functions.” But science as well as daily life furnishes examples. When it is said by Engelmann, Hensen, Hertwig, and Maupas that “the essential end of sexuality is rejuvenescence, that is, the restoration of growth-energy,” we have another instance of an explanation which explains nothing. What is the phenomenon to be explained? That unfolding of an organism from a germ which displays growth-energy. And what is the explanation? The giving of fresh growth-energy. The unknown quantity “growth-energy” is contained in the explanation proposed. There exists no conception of “juvenescence” save that derived from observing developing plants and animals; and if “re” be prefixed, no interpretation is thereby given to the unexplained thing “juvenescence.”

Coleridge somewhere comments on a source of fallacy which he calls the “hypostasis of a relation”—the changing of a relation into a thing. The plumber who tells you that water rises in a pump “by suction” supplies an instance. Having assumed suction to be an agent, he thinks that he understands how the piston does its work. Some of the explanations given of fertilization supply further instances. When it is said that sexual union has for its end “to give increased vigour to all the vital processes,” it is tacitly implied that vigour is a something—a something which can be given. But now, in the first place, it is only by the hypostasis of a relation that we are led to think of vigour as a thing. Vigour is a state—that state of a living body which enables it to give out much motion. What enables it to do this? The presence in it of abundant molecules containing much molecular motion which can be transformed into molar motion: the transformation being effected by the falling of these molecules into their simpler and relatively-inert components, which are thereupon excreted. Energy-containing matter is used up, and more energy or vigour can be given only by supplying more such matter. How then can the union of two nuclei—those of the sperm-cell and germ-cell—give vigour? Only an infinitesimal portion of vigour in the sense above explained exists in either, and the union of them leaves it still infinitesimal. And then, even supposing the vigour to be an entity and to be appreciable in quantity, how could it go on producing that immense combination of physiological actions seen in the unfolding of the germ into an organism? and how could it go on producing the physiological actions of an adult organism during a whole century?

May we not then say that these proposed explanations leave the question where it was—are nominal solutions, not real solutions?

5. But the hypothesis of constitutional units furnishes, if not a satisfactory answer yet, something in the nature of an answer—a true cause; that is to say, a cause actually known to us as operating in other cases. In § 92 it was pointed out that in proportion as units are similar, there may be built up from them an aggregate which is relatively stable, and that along with increasing dissimilarity the stability of the aggregate decreases. It was inferred that if a group of constitutional units belonging to one individual which have become moulded into relatively exact congruity with the organism and with one another by long co-operation, are mingled with some belonging to another individual which, differently circumstanced, has become somewhat different in itself and in its units, then the mass formed by the union of the two groups will be relatively unstable—relatively modifiable by incident forces. Whereas in either organism, no longer perpetually changed in the relations of its parts by growth, there is an approach towards equilibrium between the whole and its components, the components contributed by the two to form a germ, being slightly unlike one another, will not form a group in a state of equilibrium. The group they form will be capable of easy change by incident forces; and they will so be rendered free to follow their proclivities towards the typical form of the species. Inferring this we must also infer that so long as these two sets of slightly different units are not exposed to any constant forces tending to coerce them into the same form, there will continue to exist in the nuclei of all descendant cells this same relative instability and consequent plasticity.

Such evidence as we have verifies this interpretation. There is first the universal fact that development of the germ begins when it is exposed to an incident force—heat—the undulations of which, increasing the oscillations of the mixed units, give them greater freedom to arrange themselves in conformity with their type. We see this alike when spring warmth makes a seed germinate and when the warmth of a sitting hen sets up organization in her eggs. Heat frees the molecules of inorganic matter from local restraints and, as we see in molten metal, lets them yield to other forces; and similarly in this organic matter, the units are made free to follow their proclivities. Then, secondly, there comes the evidence from comparisons between the effects of mixing constitutional units differing in various degrees. Let the cluster of mixed units be derived from animals that are ordinally distinct. Nothing happens. The units each contributes tend to arrange themselves after the parental type. Hence a conflict between the tendencies towards two markedly unlike structures, and no structure arises. Suppose the mixed units come from two kindred species—say horse and ass. The structures which they respectively tend to form, being in their main characters alike, there is such co-operation as produces a working organism but an organism in certain respects imperfect—a mule. Suppose, again, the units come from two varieties of the same species. A perfect organism results, and, as shown by Mr. Darwin when detailing the effects of crossing, an unusually vigorous organism. The units being more unlike than those belonging to the same variety, the instability of the germ-plasm is unusually great, and the transformations which constitute development and action become unusually active. When, as in ordinary cases, the units are supplied by members of the same variety who have not been made very much alike by their antecedents, there follows the usual amount of organic vigour. Coming now to the results of breeding in-and-in—breeding between individuals whose constitutions (i.e. constitutional units) have for generations been growing more alike in the absence of crossing with other stirps—we see that diminution of organic vigour is displayed: there is a decrease in the rate of physiological change. Finally, on coming to a closer relationship, as in marriages between cousins, in whom the constitutional units are more than commonly alike, we see there frequently follows either barrenness or the production of feeble offspring.

All these facts, then, are congruous with the hypothesis that the use of fertilization is the mixing of unlike units, and consequent production of plasticity. Leaving out cases in which the unlikenesses are so great as wholly to prevent co-operation among the units, the degree of vigour, that is, the activity of physiological change, is great where the unlikeness is great and diminishes with the approach towards likeness.

6. The existence of constitutional units seems otherwise necessarily implied. I refer to the fact that no organism is a homogeneous mean between its parents but consists of a mixture of parts, some following one parent and some the other. Among illustrations of this the most conspicuous are those yielded by the variously-mixed colours of hair or feathers. Horses, cattle, dogs, cats, hens, pigeons display these mixtures: colours in one place like the mother and in another place like the father. As the internal organs are invisible, and as visible organs have indefinite shapes and graduate indefinitely into adjacent ones, the mixture of traits is elsewhere less conspicuous; but occasional marked cases (especially in malformations) leave no doubt that it pervades the entire organism.

This peculiarity of transmission seems necessarily to imply that there are distinct units derived from the two parents, and that in the course of development there is more or less segregation of them—those of the one origin predominating so far in some places as to give special likeness to one parent, and those derived from the other doing the like in other places. All which interpretation is impossible unless the hypothesis of constitutional units be admitted.

7. I come at length to the special evidence referred to at the outset. It is evidence of the same nature as that just assigned, but carried to a higher stage. It is furnished not by the segregation of traits derived from two parents of the same variety, but is furnished by the segregation of traits derived from parents of different varieties. In articles on “Bud Variations or Sports” (Gardener’s Chronicle, 1891) Dr. Masters gives various examples of the separation or unmixing of ancestral constitutions. Mr. Noble formed a hybrid between Clematis Jackmani and C. patens. One of these varieties flowers in the autumn on new wood, while the other flowers in the spring on old wood; and the result is that flowers of two kinds, quite unlike, are produced at different parts of the year, and that by pruning so as to cut away one or other set of shoots, the plant may be made to produce exclusively for the time being one or other sort of flower.

Thus, however the germ-plasm is constituted its essential components cannot be all alike. Before there can be this dissociation of ancestral characters, there must be in the germ-plasm different elements capable of being dissociated. This single fact seems to compel us to assume constitutional units.