Interpretation of the forms of organisms and the forms of their parts, must depend mainly on the conclusions previously drawn respecting their phylogeny; and the drawing of such conclusions must be guided by recognition of the various factors of Evolution, as well as by recognition of certain extremely general results of Evolution and certain concomitants of Evolution.
A primary one among these is that no existing species can exhibit more than approximately the ancestral structure of any other existing species. As all ancestors have disappeared, so, in a greater or less degree, the traits, specific, generic, or ordinal, which distinguished the earlier of them have disappeared. Setting out with the familiar symbol, a tree, let us regard its peripheral twigs as representing extant species; let us assume that the interior of the tree is filled up with some supporting substance, leaving only the ends of the living twigs projecting; and let us suppose the trunk, main branches, secondary branches, tertiary branches, &c., have decayed away. Then if we take these decayed parts to stand for the divergent and re-divergent lines of evolution which are represented by fossils in the Earth’s crust, it will be manifest, first, that no one of the living superficial twigs (or species) exhibits the ancestral organization whence any other of the living superficial twigs (or species) has been developed; it will be manifest, second, that the generic structure inherited by any existing species must be a structure out of which came sundry allied species—the fork, as it were, at which adjacent twigs diverged; and third, that the ancestor of an order must, in like manner, be sought at some point deeper down in the symbolic tree—a place of divergence of the sub-branches representing allied genera. Similarly with the ancestral types of classes, still deeper down in the tree or further back in time. So that phylogeny becomes more and more speculative as its questions become more and more radical. And the difficulty is made greater by the deficiency of palæontological evidence.
One obvious corollary is that an ancestral type from which sundry allied types now existing diverged, was, speaking generally, simpler than these; since the divergent types became different by the superposing of modifications, adding to their complexities. There is a further reason for inferring that the least specialized member of any group is more like the remote ancestor than any of the others; for every adaptation stands in the way of subsequent re-adaptations: it presents a greater amount of structure to be undone. To get some idea of the ancestral type where no extant member of the group is manifestly simpler than the rest, the method must be to take all its extant members and, after letting their differences mutually cancel, observe what remains common to them all.
But there are difficulties standing in the way of phylogeny, and consequently of morphology, much greater than these. Returning to our symbolic tree, it is clear that it would be far from easy to say of any one twig which extinct sub-branch, branch, and main branch it belonged to, even supposing that the growths of all parts had been uniformly outwards. Immensely more perplexing, then, must be the affiliation if various of the branches, sub-branches, &c., have sent out backward-growing shoots which have come to the surface only after prolonged retrograde courses, and if other branches have sent shoots into regions occupied by alien branches—shoots bearing twigs which come to the surface along with those to which they are but remotely allied. The problems of origin and of structure which organisms present, are met by both of the difficulties thus symbolized.
One of them arises from the prevalence of retrograde metamorphoses. Throughout the animal world these are variously displayed by parasites, multitudinous in their kinds; for most of them belong to types much higher in organization. Changed habits and consequent changed structures have so transferred them that only by study of their embryonic stages can their kinships be made out. And these retrograde metamorphoses, conspicuous among parasites, have, in the course of evolution, affected some members of all groups; for in all groups the struggle for existence has compelled some to adopt careers less trying but less profitable.
Not only by forcing on many kinds of organisms simpler ways of living, and consequent degeneracy, has the universal competition caused obscuring transformations. It has done this also by tempting many other kinds of organisms to adopt ways of life not simpler than before but merely different. Pressure continually prompts every type to intrude on other types’ spheres of activity; and so causes it to assume certain structural characters of the types whose spheres it invades, masking its previous characters. Modifications hence arising have, in the great mass of cases, been superposed one on another time after time. The aquatic animal becomes through several transitions a land-animal, and then the land-animal through other transitions becomes now an aërial animal like the bat and now an aquatic animal like the whale. Certain kinds of birds furnish extreme illustrations. There was the change from the fish to the water-breathing amphibian and then to the air-breathing amphibian; thence to the reptile living on the Earth’s surface; thence to the flying reptile and the bird; then came the diving birds, joining with their aërial life a life passed partly in the water; and finally came a type like the penguin, in which the power of flight has been lost and the water has again become the almost exclusive medium, except for breathing. Of course the mouldings and re-mouldings of structure resulting from these successive unlike modes of life, in many cases put great difficulties in the way of ascertaining which are the original corresponding parts. Some parts have become abnormally large; others have dwindled or disappeared; and the relative positions of parts have often been greatly changed. A bat’s wing and a bird’s wing are analogous organs, but their frameworks are but partially homologous. While in the bird the terminal parts of the fore-limb do little towards supporting the wing, in the bat the wing is mainly supported by enormously-developed terminal parts.
The effects of the struggle to survive, which here prompts a simpler life with resulting degeneracy and there a different life with resulting new developments, are far from being the only causes of morphological obscurations. Fulfilment of certain highly general requirements gives certain common traits to plants of widely divergent classes; and fulfilment of certain other highly general requirements gives certain common traits to animals of widely divergent classes. It was remarked in the first volume ([§ 54f]) that the cardinal distinction between the characters of plants and animals arises from the fact that while the chief food of plants is universally present the food of animals is scattered. Here it has to be added that to utilize the universally distributed food the ordinary plant needs the aid of light, and has to acquire structures enabling it to get that aid; while the ordinary animal, to utilize the scattered food, must acquire the structures needful for locomotion. Let us contemplate separately the traits hence resulting in the vegetal world and the traits hence resulting in the animal world.
The familiar plantain meets the requirement by growing stiff leaves enabling it to press down the competing grasses around which would else shade it; but the great majority of ordinary plants meet the requirement by raising themselves into the air. Hence the need for a stem, and hence the fact that plants of widely unlike natures similarly form stems which, in achieving strength enough to support the foliage and resist the wind, acquire certain adaptive structures having a general similarity. Here from the edge of a pool is a reed, and here from the adjacent copse is a hemlock: the one having grown tall in escaping the shade of its companions and the other in escaping the shade of the surrounding brushwood. On being cut across each discloses a tube, and each exhibits septa dividing this tube into chambers. In either case by the tubular structure is gained the greatest strength with the least material; but there is no morphological kinship between the tubes nor between the septa. Still more marked is the simulation of homology by analogy in another plant which the adjacent ditch may furnish—the common Horsetail. In this, again, we see an elongated vertical-growing part, raising the foliage into the air; and, as before, this is tubular and divided by septa. A type utterly alien from the other two has, by survival of the fittest, been similarly moulded to meet mechanical needs.
Passing now to the obscurations in the animal world caused by alterations favouring locomotion, we note first that the locomotive power is at the outset very slight. Among many orders of Protozoa, as also among many low types of Metazoa, vibratile cilia are the most general agents of locomotion—necessarily feeble locomotion. Regarded in the mass, the Cœlenterata, when not stationary like the Hydra or higher types in the hydroid stage, usually possess only such small self-mobility as the slow rhythmical contractions of their umbrella-disks effect, or else such as is effected by bands of cilia or of vibratile plates, as in the Beroe. Even among these low tpes of Metazoa, however, in which ordinarily the radial structure is conspicuous, or but slightly obscured by an ovoid form as in the Ctenophora, we find, in the Cestus veneris, extreme obscuration caused by an elongation which facilitates movement through the water; alike by the actions of its vibratile plates and by its undulations, which simulate those of sundry higher animals.
And here we come upon the essential fact to be recognized. Elongation favours locomotion in various ways that are severally taken advantage of by different types of creatures. (1) To a given mass of moving matter the resistance of the medium decreases along with decrease in the area of its transverse section, and this implies increase of length: a given force will move the lengthened mass along with greater facility. (2) Reaching a certain point the elongated form enables an animal to progress by undulations, as in the water fish do, and even some cœlenterates and turbellarians do, and as on land snakes do: lateral resistances serving in either case as fulcra. (3) Lengthening of the body serves otherwise to aid locomotion in the creeping or burrowing worm, which, utilizing the statical resistance of its hinder part thrusts onwards its fore part, and then, holding fast its fore part by the aid of minute setæ, draws the hinder part after it. But elongation, doubly advantageous at first, while the body is itself the chief instrument of locomotion, gradually loses its advantageousness as special instruments of locomotion are developed. (4) This we see in that locomotive action effected by limbs, which, many and small in the lower Arthropoda and becoming few and larger in the higher, at length give great activity to a shortened and consolidated body: a stage reached only through stages of decreasing elongation accompanying increase of limb-power. (5) In the Vertebrata locomotion by undulations comes, along certain lines of evolution, to be replaced by that limb locomotion which accompanies the rise from water-life to land-life: the evolution of Amphibians exhibiting the transition. (6) Further, we see among mammals that as limbs become efficient the elongated body ceases to be itself instrumental in locomotion, but that still some elongation remains a characteristic. (7) Finally, where limb locomotion reaches its highest degree, as in birds, elongation disappears.