§ 244. Certain of the Protozoa are quite indefinite in their shapes, and quite inconstant in those indefinite shapes which they have—the relations of their parts are indeterminate both in space and time. In one of the simpler Rhizopods, at least during the active stage of its existence, no permanent distinction of inside and outside is established; and hence there can arise no established correspondence between the shape of the outside and the distribution of environing actions. But when the relation of inner and outer becomes fixed, either over part of the mass or over the whole of it, we have kinds of symmetry that correspond with the habitual incidence of forces. An Amœba in becoming encysted, passes from an indefinite, ever-changing form into a spherical form; and the order of symmetry which it thus assumes, is in harmony with the average equality of the actions on all its sides. In Difflugia, Fig. 134, and still better in Arcella, we have an indefinitely-radial symmetry occurring where the conditions are different above and below but alike all around. Among the Gregarinida the spherical symmetry and symmetry passing from that into the radial, are such as appear to be congruous with the simple circumstances of these creatures in the intestines of insects. But the relations of these lowest types to their environments are comparatively so indeterminate, and our knowledge of their actions so scanty, that little beyond negative evidence can be expected from the study of them.

The like may be said of the Infusoria. These are more or less irregular. In some cases, where the line of movement through the water is tolerably definite and constant, we have a form that is approximately radial—externally at least. But usually, as shown in Figs. 137, 138, 139, there is either an unsymmetrical or an asymmetrical shape. And when one of these creatures is watched under the microscope, the congruity of this shape with the incidence of forces is manifest. For the movements are conspicuously varied and indeterminate—movements which do not expose any two or more sides of the mass to approximately equal sets of actions.[43]

§ 245. Among aggregates of the second order, as among aggregates of the first order, we find that of those possessing any definite shapes the lowest are spherical or spheroidal. Such are some of the Radiolaria, as Collozoum inerme. These bodies which float passively in the sea, and present in turn all their sides to the same influences, have their parts disposed with approximate regularity round a centre—approximate, because in the absence of locomotion a slight irregularity of growth, almost certain to take place, may cause a fixed attitude and a resulting deviation from spherical symmetry. The best cases in illustration of the truth here named, are furnished by rotating and locomotive organisms respecting which there is a dispute whether they are animal or vegetal—the Volvocineæ. These, already instanced under the one head in § 218, may here be instanced afresh under the other. Further, among these secondary aggregates in which the units, only physically integrated, have not had their individualities merged into an individuality of a higher order, must be named the compound Infusoria. The cluster of Vorticellæ in Fig. 144, will sufficiently exemplify them; and the striking resemblance borne by its individuals to those of a radially-arranged cluster of flowers, will show how, under analogous conditions, the general principles of morphological differentiation are similarly illustrated in the two kingdoms.

§ 246. Radial symmetry is usual in low aggregates of the second order which have their parts sufficiently differentiated and integrated to give individualities to them as wholes. The Cœlenterata offer numerous examples of this. Solitary polypes—hydroid or helianthoid—mostly stationary, and when they move, moving with any side foremost, do not by locomotion subject their bodies to habitual contrasts of conditions. Seated with their mouths upwards or downwards, or else at all degrees of inclination, the individuals of a species taken together, are subject to no mechanical actions affecting some parts of their discs more than other parts. And this indeterminateness of attitude similarly prevents their relations to prey from being such as subject some of their prehensile organs to forces unlike those to which the rest are subject. The fixed end is differently conditioned from the free end, and the two are therefore different; but around the axis running from the fixed to the free end the conditions are alike in all directions, and the form therefore is radial. Again, among many of the simple free-swimming Hydrozoa, the same general truth is exemplified under other circumstances. In a common Medusa, advancing through the water by the rhythmical contractions of its disc, the mechanical reactions are the same on all sides; and as, from accidental causes, every part of the edge of the disc comes uppermost in its turn, no part is permanently affected in a different way from the rest. Hence the radial form continues.

In others of this same group, however, there occur forms which show us an incipient bilateralness; and help us to see how a more decided bilateralness may arise. Sundry of the Medusidæ are proliferous, giving origin to gemmæ from the body of the central polypite or from certain points on the edge of the disc; and this budding, unless it occurs equally on all sides, which it does not and is unlikely to do, must tend to destroy the balance of the disc, and to make its attitude less changeable. In other cases the growth of a large process [a much-developed tentacle] from the edge of the disc on one side, as in Steenstrupia, Fig. 257, constitutes a similar modification, and a cause of further modification. The animal is no longer divisible into any two quite similar halves, except those formed by a plane passing through the process; and unless the process is of the same specific gravity as the disc, it must tend towards either the lowest or the highest point, and must so serve to increase the bilateralness, by keeping the two sides of the disc similarly conditioned while the top and bottom are differently conditioned. Fig. 258 represents the underside of another Medusa, in which a more decided bilateralness is produced by the presence of two such processes. Among the simple free-swimming Actinozoa, occur like deviations from radial symmetry, along with like motions through the water in bilateral attitudes. Of this a Cydippe is a familiar example. Though radial in some of its characters, as in the distribution of its meridional bands of locomotive paddles with their accompanying canals, this creature has a two-sided distribution of tentacles and various other parts, corresponding with its two-sided attitude in moving through the water. And in other genera of this group, as in Cestum, Eurhamphæa, and Callianira, that almost equal distribution of parts which characterizes the Beroe is quite lost.

Here seems a fit place to meet the objection which some may feel to this and other such illustrations, that they amount very much to physical truisms. If the parts of a Medusa are disposed in radial symmetry round the axis of motion through the water, there will of course be no means of maintaining one part of its edge uppermost more than another; and the equality of conditions may be ascribed to the radiateness, as much as the radiateness to the equality of conditions. Conversely, when the parts are not radially arranged around the axis of motion, they must gravitate towards some one attitude, implying a balance on the two sides of a vertical plane—a bilateralness; and the two-sided conditions so necessitated, may be as much ascribed to the bilateralness as the bilateralness to the two-sided conditions. Doubtless the form and the conditions are, in the way alleged, necessary correlates; and in so far as it asserts this, the objection harmonizes with the argument. To the difficulty which it at the same time raises by the implied question—Why make the form the result of the conditions, rather than the conditions the result of the form? the reply is this:—The radial type, both as being the least differentiated type and as being the most obviously related to lower types, must be taken as antecedent to the bilateral type. The individual variations which incidental circumstances produce in the radial type, will not cause divergence of a species from the radial type, unless such variations give advantages to the individuals displaying them; which there is no reason to suppose they will always do. Those occasional deviations from the radial type, which the law of the instability of the homogeneous warrants us in expecting to take place, will, however, in some cases be beneficial; and will then be likely to establish themselves. Such deviations must tend to destroy the original indefiniteness and variability of attitude—must cause gravitation towards an habitual attitude. And gravitation towards an habitual attitude having once commenced, will continually increase, where increase of it is not negatived by adverse agencies: each further degree of bilateralness rendering more decided the actions that conduce to bilateralness. If this reply be thought insufficient, it may be enforced by the further one, that as, among plants, the incident forces are the antecedents and the forms the consequents (changes of forces being in many cases visibly followed by changes of forms) we are warranted in concluding that the like order of cause and effect holds among animals.[44]

§ 247. Keeping to the same type but passing to a higher degree of composition, we meet more complex and varied illustrations of the same general laws. In the compound Cœlenterata, presenting clusters of individuals which are severally homologous with the solitary individuals last dealt with, we have to note both the shapes of the individuals thus united, and the shapes of the aggregates made up of them.

Such of the fixed Hydrozoa and Actinozoa as form branched societies, continue radial; both because their varied attitudes do not expose them to appreciable differences in their relations to those surrounding actions which chiefly concern them (the actions of prey), and because such differences, even if they were appreciable, would be so averaged in their effects on the dissimilarly-placed members of each group as to be neutralized in the race. Among the tree-like coral-polypedoms, as well as in such ramified assemblages of simpler polypes as are shown in Figs. 149, 150, we have, indeed, cases in many respects parallel to the cases of scattered flowers (§ 233), which though placed laterally remain radial, because no differentiating agency can act uniformly on all of them. Meanwhile, in the groups which these united individuals compose, we see the shapes of plants further simulated under a further parallelism of conditions. The attached ends differ from the free ends as they do in plants; and the regular or irregular branches obviously stand to environing actions in relations analogous to those in which the branches of plants stand.

The members of those compound Cœlenterata which move through the water by their own actions, in attitudes that are approximately constant, show us a more or less distinct two-sidedness. Diphyes, Fig. 259, furnishes an example. Each of the largely-developed and modified polypites forming its swimming sacs is bilateral, in correspondence with the bilateralness of its conditions; and in each of the appended polypites the insertion of the solitary tentacle produces a kindred divergence from the primitive radial type. The aggregate, too, which here very much subordinates its members, exhibits the same conformity of structure to circumstances. It admits of symmetrical bisection by a plane passing through its two contractile sacs, or nectocalyces, but not by any other plane; and the plane which thus symmetrically bisects it, is the vertical plane on the two sides of which its parts are similarly conditioned as it propels itself through the water.

Another group of the oceanic Hydrozoa, the Physophoridæ, furnishes interesting evidence—not so much in respect of the forms of the united individuals, which we may pass over, as in respect of the forms of the aggregates. Some of these are without swimming organs, and have their parts suspended from air-vessels which habitually float on the surface of the water. Hence the distribution of their parts is asymmetrical. The Physalia, Fig. 152, is an example. Here the relations of the integrated group of individuals to the environment are indefinite; and there is thus no agency tending to change that comparatively irregular mode of growth which is probably derived from a primordial type of the branched Hydrozoa.

So various are the modes of union among the compound Cœlenterata, that it is out of the question to deal with them all. Even did space permit, it would be impracticable for any one but a professed naturalist, to trace throughout this group the relations between shapes and conditions of existence. The above must be taken simply as a few of the most significant and easily-interpretable cases.

§ 248. In the sub-kingdoms Polyzoa and Tunicata we meet with examples not wholly unlike the foregoing. Among the types assembled under these names there are simple individuals or aggregates of the second order, and societies or tertiary aggregates produced by their union. The relations of forms to forces have to be traced in both.

Solitary Ascidians, fixed or floating, carry on an inactive and indefinite converse with the actions in the environment. Without power to move about vivaciously, and unable to catch any prey but that contained in the currents of water they absorb and expel, these creatures are not exposed to sets of forces which are equal on two or more sides; and their shapes consequently remain vague. Though internally their parts have a partially-symmetrical arrangement, due to their derivation, yet they are substantially unsymmetrical in that part of the body which is concerned with the environment. Fig. 156 is an example.[45] Among the composite Ascidians, floating and fixed, the shape of the aggregate, partly determined by the habitual mode of gemmation and partly by the surrounding conditions in each case, is in great measure indefinite. We can say no more about it than that it is not obviously at variance with the laws alleged.

Evidence of a more positive kind occurs among those compound Molluscoida which are most like the compound Cœlenterata in their modes of union—the Polyzoa. Many of these form groups that are more or less irregular—spreading as films over solid surfaces, combining into seaweed-like fronds, budding out from creeping stolons, or growing up into tree-shaped societies; and besides aggregating irregularly they are irregularly placed on surfaces inclined in all directions. Merely noting that this asymmetrical distribution of the united individuals is explained by the absence of definiteness in the relations of the aggregate to incident forces, it concerns us chiefly to observe that the united individuals severally exemplify the same truth as do similarly-united individuals among the Cœlenterata. Averaging the members of each society, the ciliated tentacles they protrude are similarly related to prey on all sides; and therefore remain the same on all sides. This distribution of tentacles is not, however, without exception. Among the fresh-water Polyzoa there are some genera, as Plumatella and Crystatella, in which the arrangement of these parts is very decidedly bilateral. Some species of them show us such relations of the individuals to one another and to their surface of attachment, as give a clue to the modification; but in other species the meaning of this deviation from the radial type is not obvious.

§ 249. In the Platyhelminthes good examples of the connexions between forms and forces occur. The Planaria exemplifies the single bilateral symmetry which, even in very inferior forms, accompanies the habit of moving in one direction over a solid surface. Humbly organized as are these creatures and their allies the Nemertidæ, we see in them, just as clearly as in the highest animals, that where the movements subject the body to different forces at its two ends, different forces on its under and upper surfaces, and like forces along its two sides, there arises a corresponding form, unlike at its extremities, unlike above and below, but having its two sides alike.

The Echinodermata furnish us with instructive illustrations—instructive because among types that are nearly allied, we meet with wide deviations of form answering to marked contrasts in the relations to the environment. The facts fall into four groups. The Crinoidea, once so abundant and now so rare, present a radial symmetry answering to an incidence of forces that are equal on all sides. In the general attitudes of their parts towards surrounding actions, they are like uniaxial plants or like polypes; and show, as those do, marked differences between the attached ends and the free ends, along with even distributions of parts all round their axes. In the Ophiuridea, and in the Star-fishes, we have radial symmetry co-existing with very different habits; but habits which nevertheless account for the maintenance of the form. Holding on to rocks and weeds by its simple or branched arms, or by the suckers borne on the under surface of its rays, one of these creatures moves about not always with one side foremost, but with any side foremost. Consequently, averaging its movements, its arms or rays are equally affected, and therefore remain the same on all sides. On watching the ways of the common Sea-urchin, we are similarly furnished with an explanation of its spherical, or rather its spheroidal, figure. Here the habit is not to move over any one approximately-flat surface; but the habit is to hold on by several surfaces on different sides at the same time. Frequenting crevices and the interstices among stones and weeds, the Sea-urchin protrudes the suckers arranged in meridional bands over its shell, laying hold of objects now on this side and now on that, now above and now below: the result being that it does not move in all directions over one plane but in all directions through space. Hence the approach in general form towards spherical symmetry—an approach which is, however, restrained by the relations of the parts to the mouth and vent: the conditions not being exactly the same at the two poles as at other parts of the surface. Still more significant is that deviation from this shape which occurs among such of the Echinidea as have habitats of a different kind, and consequently, different habits. The genera Echinocyamus, Spatangus, Brissus, and Amphidotus, diverge markedly towards a bilateral structure. These creatures are found not on rocky shores but on flat sea-bottoms, and some of them only on bottoms of sand or mud. Here, there is none of that distribution of surfaces on all sides which makes the spheroidal form congruous with the conditions. Having to move about over an approximately-horizontal plane, any deviation of structure arising accidentally which leads to one side being kept always foremost, will be an advantage: greater fitness to function becoming possible in proportion as function becomes fixed. Survival of the fittest will therefore tend to establish, under such conditions, a form that keeps the same part in advance—a form in which, consequently, the original radial symmetry diverges more and more towards bilateral symmetry.

§ 250. Very definite and comparatively uniform, are the relations between shapes and circumstances among the Annulosa: including under that title the Annelida and the Arthropoda. The agreements and the disagreements are equally instructive.

At one time or other of its life, if not throughout its life, every annulose animal is locomotive; and its temporary or permanent locomotion, being carried on with one end habitually foremost and one surface habitually uppermost, it fulfils those conditions under which bilateral symmetry arises. Accordingly, bilateral symmetry is traceable throughout the whole of this sub-kingdom. Traceable, we must say, because, though it is extremely conspicuous in the immense majority of annulose types, it is to a considerable extent obscured where obscuration is to be expected. The embryos of the Tubicolæ, after swimming about a while, settle down and build themselves tubes, from which they protrude their heads; and in them, or in some of them, the bilateral symmetry is disguised by the development of head-appendages in an all-sided manner. The tentacles of Terebella are distributed much in the same way as those of a polype. The breathing organs in Sabella unispira, Fig. 260, do not correspond on opposite sides of a median plane. Even here, however, the body retains its primitive bilateralness; and it is further to be remarked that this loss of bilateralness in the external appendages, does not occur where the relations to external conditions continue bilateral: witness the Serpula, Fig. 261, which has its respiratory tufts arranged in a two-sided way, under the two-sided conditions involved by the habitual position of its tube.

The community of symmetry among the higher Annulosa, has an unobserved significance. That Flies, Beetles, Lobsters, Centipedes, Spiders, Mites, have in common the characters, that the end which moves in advance differs from the hinder end, that the upper surface differs from the under surface, and that the two sides are alike, is a truth received as a matter of course. After all that has been said above, however, it will be seen to have a meaning not to be overlooked; since it supplies a million-fold illustration of the laws which have been set forth. It is needless to give diagrams. Every reader can call to mind the unity indicated.

While, however, annulose animals repeat so uniformly these traits of structure, there are certain other traits in which they are variously contrasted; and their contrasts have to be here noted, as serving further to build up the general argument. In them we see the stages through which bilateral symmetry becomes gradually more marked, as the conditions it responds to become more decided. A common Earth-worm may be instanced as a member of this sub-kingdom that is among the least-conspicuously bilateral. Though internally its parts have a two-sided arrangement; and though the positions of its orifices give it an external two-sidedness, at the same time that they establish a difference between the two ends; yet its two-sidedness is not strongly-marked. The form deviates but little from what we have distinguished as triple bilateral symmetry: if the creature is cut across the middle, the head and tail ends are very much alike; if cut in two along its axis by a horizontal plane, the under and upper halves are very much alike, externally if not internally; and if cut in two along its axis by a vertical plane, the two sides are quite alike. Figs. 263 and 264 will make this clear. Such creatures as the Julus and the Centipede, may be taken as showing a transition to double bilateral symmetry. Besides being divisible into exactly similar halves by a vertical plane passing through its axis, one of these animals may be bisected transversely into parts that differ only slightly; but if cut in two by a horizontal plane passing through its axis, the under and upper halves are decidedly unlike. Figs. 265, 266, exhibit these traits. Among the isopodous crustaceans, the departure from these low types of symmetry is more marked. As shown in Figs. 267 and 268, the contrast between the upper and under parts is greater, and the head and tail ends differ more obviously. In all the higher Arthropoda, the unlikeness between the front half and the hind half has become conspicuous. There is in them single bilateral symmetry of so pronounced a kind, that no other resemblance is suggested than that between the two sides. By Figs. 269 and 270, representing a decapodous crustacean divided longitudinally and transversely, this truth is made manifest. On calling to mind the habits of the creatures here drawn and described, it will be seen that they explain these forms. The incidence of forces is the same all around the Earth-worm as it burrows through the compact ground. The Centipede, creeping amid loose soil or débris or beneath stones, insinuates itself between solid surfaces—the interstices being mostly greater in one dimension than in others. And all the higher Annulosa, moving about as they do over exposed objects, have their dorsal and ventral parts as dissimilarly acted upon as are their two ends.

One other fact only respecting annulose animals needs to be noticed under this head—the fact, namely, that they become unsymmetrical where their parts are unsymmetrically related to the environment. The common Hermit-crab serves as an instance. Here, in addition to the unlikeness of the two sides implied by that curvature of the body which fits the creature to the shell it inhabits, there is an unlikeness due to the greater development of the limbs, and especially the claws, on the outer side. As in the embryo of the Hermit-crab the two sides are alike; and as both the embryo and the ancestor lived in such a way, being free, that the conditions were alike on the two sides; and as the embryo may be taken to represent the type from which the Hermit-crab has been derived; we have in this case evidence that a symmetrically-bilateral form has been moulded into an unsymmetrically-bilateral form, by the action of unsymmetrically-bilateral conditions. A further illustration is supplied by Bopyrus, Fig. 271: a parasite which lives in the branchial chamber of prawns, and whose habits similarly account for its distorted shape.

§ 251. Among the Mollusca we find more varied relations between shapes and circumstances. Some of these relations are highly instructive.

Mollusks of one order, the Pteropoda, swim in the sea much in the same way that butterflies fly in the air, and have shapes not altogether unlike those of butterflies. Fig. 272 represents one of these creatures. That its bilaterally-symmetrical shape harmonizes with its bilaterally-symmetrical conditions is sufficiently obvious.

Among the Lamellibranchiata, we have diverse forms accompanying diverse modes of life. Such of them as frequently move about, like the fresh-water Mussel, have their two valves and the contained parts alike on the opposite sides of a vertical plane: they are bilaterally symmetrical in conformity with their mode of movement. The marine Mussel, too, though habitually fixed, and though not usually so fixed that its two valves are similarly conditioned, still retains that bilateral symmetry which is characteristic of the order; and it does this because in the species considered as a whole, the two valves are not dissimilarly conditioned. If the positions of the various individuals are averaged, it will be seen that the differentiating actions neutralize one another. In certain other fixed Lamellibranchs, however, there is a considerable deviation from bilateral symmetry; and it is a deviation of the kind to be anticipated under the circumstances. Where one valve is always downwards, or next to the surface of attachment, while the other valve is always upwards, or next to the environing water, we may expect to find the two valves become unlike. This we do find: witness the Oyster. In the Oyster, too, we see a further irregularity. There is a great indefiniteness of outline, both in the shell and in the animal—an indefiniteness made manifest by comparing different individuals. We have but to remember that growing clustered together, as Oysters do, they must interfere with one another in various ways and degrees, to see how the indeterminateness of form and the variety of form are accounted for.

Among the Gasteropods modifications of a more definite kind occur. “In all Mollusks,” says Professor Huxley, “the axis of the body is at first straight, and its parts are arranged symmetrically with regard to a longitudinal vertical plane, just as in a vertebrate or an articulate embryo.” In some Gasteropods, as the Chiton, this bilateral symmetry is retained—the relations of the body to surrounding actions not being such as to disturb it. But in those more numerous types which have spiral shells, there is a marked deviation from bilateral symmetry, as might be expected. “This asymmetrical over-development never affects the head or foot of the mollusk”: only those parts which, by inclosure in a shell, are protected from environing actions, lose their bilateralness; while the external parts, subjected by the movements of the creatures to bilateral conditions, remain bilateral. Here, however, a difficulty meets us. Why is it that the naked Gasteropods, such as our common slugs, deviate from bilateral symmetry, though their modes of movement are those along with which complete bilateral symmetry usually occurs? The reply is that their deviations from bilateral symmetry are probably inherited, and that they are maintained in such parts of their organization as are not exposed to bilaterally-symmetrical conditions. There is reason to believe that the naked Gasteropods are descended from Gasteropods which had shells: the evidence being that the naked Gasteropods have shells during the early stages of their development, and that some of them retain rudimentary shells throughout life. Now the shelled Gasteropods deviate from bilateral symmetry in the disposition of both the alimentary system and the reproductive system. The naked Gasteropods, in losing their shells, have lost that immense one-sided development of the alimentary system which fitted them to their shells, and have acquired that bilateral symmetry of external figure which fits them for their habits of locomotion; but the reproductive system remains one-sided, because, in respect to it, the relations to external conditions remain one-sided.

The Cephalopods show us bilaterally-symmetrical external forms along with habits of movement through the water in two-sided attitudes. At the same time, in the radial distribution of the arms, enabling one of these creatures to take an all-sided grasp of its prey, we see how readily upon one kind of symmetry there may be partially developed another kind of symmetry, where the relations to conditions favour it.

§ 252. The Vertebrata illustrate afresh the truths which we have already traced among the Annulosa. Flying through the air, swimming through the water, and running over the earth as vertebrate animals do, in common with annulose animals, they are, in common with annulose animals, different at their anterior and posterior ends, different at their dorsal and ventral surfaces, but alike along their two sides. This single bilateral symmetry remains constant under the extremest modifications of form. Among fish we see it alike in the horizontally-flattened Skate, in the vertically-flattened Bream, in the almost-spherical Diodon, and in the greatly-elongated Syngnathus. Among reptiles the Turtle, the Snake, and the Crocodile all display it. And under the countless modifications of structure displayed by birds and mammals, it remains conspicuous.

A less obvious fact which it concerns us to note among the Vertebrata, parallel to one which we noted among the Annulosa, is that whereas the lower vertebrate forms deviate but little from triple bilateral symmetry, the deviation becomes great as we ascend. Figs. 273 and 274 show how, besides being divisible into similar halves by a vertical plane passing through its axis, a Fish is divisible into halves that are not very dissimilar by a horizontal plane passing through its axis, and also into other not very dissimilar halves by a plane cutting it transversely. If, as shown in Figs. 275 and 276, analogous sections be made of a superior Reptile, the divided parts differ more decidedly. When a Mammal and a Bird are treated in the same way, as shown in Figs. 277, 278, and Figs. 279, 280, the parts marked off by the dividing planes are unlike in far greater degrees. On considering the mechanical converse between organisms of these several types and their environments—on remembering that the fish habitually moves through a homogeneous medium of nearly the same specific gravity as itself, that the terrestrial reptile either crawls on the surface or raises itself very incompletely above it, that the more active mammal, having its supporting parts more fully developed, thereby has the under half of its body made more different from the upper half, and that the bird is subject by its mode of life to yet another set of actions and reactions; we shall see that these facts are quite congruous with the general doctrine, and furnish further support to it.

One other significant piece of evidence must be named. Among the Annulosa we found unsymmetrical bilateralness in creatures having habits exposing them to unlike conditions on their two sides; and among the Vertebrata we find parallel cases. They are presented by the Pleuronectidæ—the order of distorted flat fishes to which the Sole and the Flounder belong. On the hypothesis of evolution, we must conclude that fishes of this order have arisen from an ordinary bilaterally-symmetrical type of fish, which, feeding at the bottom of the sea, gained some advantage by placing itself with one of its sides downwards, instead of maintaining the vertical attitude. Besides the general reason there are special reasons for concluding this. In the first place, the young Sole or Flounder is bilaterally symmetrical—has its eyes on opposite sides of its head and swims in the usual way. In the second place, the metamorphosis which produces the unsymmetrical structure sometimes does not take place—there are abnormal Flounders that swim vertically, like other fishes. In the third place, the transition from the symmetrical structure to the unsymmetrical structure may be traced. Almost incredible though it seems, one of the eyes is transferred from the underside of the head to the upper side: the transfer being effected by a distorted development of the cranial bones—atrophy of some and hypertrophy of others, along with a general twist. This metamorphosis furnishes several remarkable illustrations of the way in which forms become moulded into harmony with incident forces. For besides the divergence from bilateral symmetry involved by presence of both eyes upon the upper side, there is a further divergence from bilateral symmetry involved by differentiation of the two sides in respect to the contours of their surfaces and the sizes of their fins. And then, what is still more significant, there is a near approach to likeness between the halves that were originally unlike, but are, under the new circumstances, exposed to like conditions. The body is divisible into similarly-shaped parts by a plane cutting it along the side from head to tail: “the dorsal and ventral instead of the lateral halves become symmetrical in outline and are equipoised.”

§ 253. Thus, little as there seems in common between the shapes of plants and the shapes of animals, we yet find, on analysis, that the same general truths are displayed by both. The one ultimate principle that in any organism equal amounts of growth take place in those directions in which the incident forces are equal, serves as a key to the phenomena of morphological differentiation. By it we are furnished with interpretations of those likenesses and unlikenesses of parts, which are exhibited in the several kinds of symmetry; and when we take into account inherited effects, wrought under ancestral conditions contrasted in various ways with present conditions, we are enabled to comprehend, in a general way, the actions by which animals have been moulded into the shapes they possess.

To fill up the outline of the argument, so as to make it correspond throughout with the argument respecting vegetal forms, it would be proper here to devote a chapter to the differentiations of those homologous segments out of which animals of certain types are composed. Though, among most animals of the third degree of composition, such as the rooted Hydrozoa, the Polyzoa, and the Ascidioida, the united individuals are not reduced to the condition of segments of a composite individual, and do not display any marked differentiations; yet there are some animals in which such subordinations, and consequent heterogeneities, occur. The oceanic Hydrozoa form one group of them; and we have seen reason to conclude that the Annulosa form another group. It is not worth while, however, to occupy space in detailing these unlikenesses of homologous segments, and seeking specific explanations of them. Among the oceanic Hydrozoa they are extremely varied; and the habits and derivations of these creatures are so little known, that there are no adequate data for interpreting the forms of the parts in terms of their relations to the environment. Conversely, among the Annulosa those differentiations of the homologous segments which accompany their progressing integration, have so much in common, and have general causes which are so obvious, that it is needless to deal with them at any length. They are all explicable as due to the exposure of different parts of the chain of segments to different sets of actions and reactions: the most general contrast being that between the anterior segments and the posterior segments, answering to the most general contrast of conditions to which annulose animals subject their segments; and the more special contrasts answering to the contrasts of conditions entailed by their more special habits.

Were an exhaustive treatment of the subject practicable, there should here, also, come a chapter devoted to the internal structures of animals—meaning, more especially, the shapes and arrangements of the viscera. The relations between forms and forces among these inclosed parts are, however, mostly too obscure to allow of interpretation. Protected as the viscera are in great measure from the incidence of external forces, we are not likely to find much correspondence between their distribution and the distribution of external forces. In this case the influences, partly mechanical, partly physiological, which the organs exercise on one another, become the chief causes of their changes of figure and arrangement; and these influences are complex and indefinite. One general fact may, indeed, be noted—the fact, namely, that the divergence towards asymmetry which generally characterizes the viscera, is marked among those of them which are most removed from mechanical converse with the environment, but not so marked among those of them which are less removed from such converse. Thus while, throughout the Vertebrata, the alimentary system, with the exception of its two extremities, is asymmetrically arranged, the respiratory system, which occupies one end of the body, generally deviates but little from bilateral symmetry, and the reproductive system, partly occupying the other end of the body, is in the main bilaterally symmetrical: such deviation from bilateral symmetry as occurs, being found in its most interiorly-placed parts, the ovaries. Just indicating these facts as having a certain significance, it will be best to leave this part of the subject as too involved for detailed treatment.

Internal structures of one class, however, not included among the viscera, admit of general interpretation—structures which, though internal, are brought into tolerably-direct relations with environing forces, and are therefore subordinate in their forms to the distribution of those forces. These internal structures it will be desirable to deal with at some length; both because they furnish important illustrations enforcing the general argument, and because an interpretation of them which we have seen reason to reject, cannot be rejected without raising the demand for some other interpretation.