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have ever lived on this earth, have to be classed together, and the best arrangement is like that of a genealogical tree, we may believe that descent is the hidden bond of connection which naturalists have been seeking under the term of the natural system. The facts of morphology (or the forms of living things) become intelligible, whether we look to the same pattern displayed in the homologous organs of the different species of animals (such as the hand of a man and the wing of a bat), or to the homologous parts constructed on the same pattern in each individual (as, e.g., the arm of a man and his leg). On the view that species are only strongly marked and permanent varieties, and that each species first existed as a variety of some other species, we can see why it is that naturalists have had such difficulty in defining species, and drawing lines of demarcation between species and varieties. On this view we can understand how it is that, in the eyes of most naturalists, the structure of the embryo is even more important for classification than that of the adult; for the embryo is the animal in its less modified state; and so far it reveals the structure of its progenitor. As natural selection acts by competition, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates; so that we need feel no surprise at the inhabitants of any one country, although on the ordinary view supposed to have been specially created and adapted for that country, being beaten and supplanted by the naturalized productions from another land. Rudimentary or atrophied organs, though of no use to their possessor, are retained through the force of inheritance, and are reminiscences of more fully-developed structures in the

ancestors. The fact of the fossil remains of each formation being in some degree intermediate in character between the fossils in the formations above and below, is simply explained by their intermediate position in the chain of descent. Old forms are supplanted by new ones, and neither single species nor groups of species re-appear when the chain of ordinary generation has once been broken. The grand fact that all extinct organic beings belong to the same system with recent beings, falling either into the same or into intermediate groups, follows from the living and the extinct being the offspring of common parents. As the groups which have descended from an ancient progenitor have generally diverged in character, the progenitor, with its early descendants, will often be intermediate in character in comparison with its later descendants; and thus we can see why the more ancient a fossil is the oftener it stands in some degree intermediate between existing allied groups. Lastly, the law of the long endurance of allied forms on the same continentof marsupials in Australia, of edentata in America, and other such cases-is intelligible, for within a confined country the recent and the extinct will naturally be allied by descent.1

Evolution v. Plan.-We have seen that the members of the same class, independently of their habits of life, resemble each other in the general plan of their organization: the hand of a man, formed for grasping; that of a mole, made for digging; the leg of the horse, the paddle of the porpoise, and the wing of the bat, are all constructed on the same pattern, and include similar bones in the same relative positions. Why should this

1 Origin of Species, chap. xiv.

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be, when the limbs are used for such totally different purposes? Why should the sepals, petals, stamens, and pistils in any individual flower, though fitted for such widely different purposes, be all constructed on the same pattern? Why, under the down-covered body of the moth, and under the hard wing-cases of the beetle, should there be discovered the same number of divisions as in the calcareous framework of the lobster -just twenty segments in hundreds of thousands of species? Some naturalists have been satisfied with saying that the Creator chooses to work on a certain plan of organization, an ideal type, to which He makes the structure of many creatures to conform for the sake of uniformity, even when no useful purpose is served by it. For instance, the vertebrate form in general is conceived to exist as an "idea" or " archetypal exemplar," on which it has pleased the Creator to frame certain of His creatures. But there are difficulties in the way of accepting this solution. "When we discover that the possession of seven cervical vertebræ is a general characteristic of mammals, whether the neck be immensely long, as in the giraffe, or quite rudimentary, as in the whale, shall we say that though, for the whale's neck, one vertebra would have been equally good, and though, for the giraffe's neck, a dozen would probably have been better than seven, yet seven was the number adhered to in both cases, because seven was fixed upon for the mammalian type? And then when it turns out that this possession of seven cervical vertebræ is not an absolutely universal characteristic of mammals, shall we conclude that while, in a host of cases, there is a needless adherence to a plan for the sake of consistency, there is yet, in some cases, an inconsistent abandonment of the plan?

I think we may properly refuse to draw any such conclusion."1

And with regard to segmented animals, it cannot be by chance that the dragonfly, the lady-bird, the butterfly, and the crab should each have just twenty divisions of the body (as they have, either distinctly marked, or so fused together that sometimes it is difficult to detect them); nor is there any reason to think it was necessary in the sense that no other number would have made a possible organism; and to say that the Creator followed this pattern throughout, merely for the purpose of maintaining the pattern, is to assign a motive which, if avowed by a human being, we should call whimsical. No rational interpretation of this, and hosts of like morphological truths, can be given except by the hypothesis of evolution; and from the hypothesis of evolution they are corollaries. We may accept Mr Spencer's view thus far without believing with him that the rejection of the archetypal idea is the rejection of design. The design is certainly not to give twenty segments for the sake of uniformity-they seem to be present through the force of inheritance-but can we say that there was no design to create segmented animals by evolution, and to cause the offspring to retain the parental characters?

Embryology. The embryos, or unborn offspring, of animals, within the same class, are often strikingly similar, and those of distinct classes are in their earliest stages exceedingly like one another; so much so that, except by their size, it is impossible to distinguish the embryos of mammal, bird, lizard, or snake. "In my possession," says Von Baer, "are two little embryos in spirit, whose names I have omitted to attach, and at 1 Principles of Biology, i. 309, 381.

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present I am quite unable to say to what class they belong. They may be lizards or small birds, or very young mammalia, so complete is the similarity in the mode of formation of the head and trunk in these animals. The extremities, however, are still absent in these embryos. But even if they had existed in the earliest stage of their development we should learn nothing, for the feet of lizards and mammals, the wings and feet of birds, no less than the hands and feet of man, all arise from the same fundamental form. . . . The special type is always evolved from a more general type." The special type is not always completely elaborated even at birth, but a trace of the law of embryonic resemblance sometimes lasts till a rather late age; thus birds of the same genus, and birds of closely allied genera, often resemble each other in their first and second plumage, and very young pigeons of widely different breeds show incomparably less proportional differences than the grown birds.

If Mr Darwin's views be correct, the forms of life in ancient geologic periods were not so distinctly separated as they are now, because, as time has gone on, the lifetree has sent out branches diverging more and more from the original trunk. In this respect there is a close correspondence between the general development of living forms and the development of a mammalian embryo at the present day. Von Baer says of the embryo, that in its earliest stage every organism has the greatest number of characters in common with all other organisms in their earliest stages; that at a stage somewhat later, its structure is like the structures displayed at corresponding phases by a less extensive multitude of organisms; that at each subsequent stage traits are