To my mind the rudimentary and vestigial structures of animals are in themselves proof positive of a natural history of change.  The few illustrations can be reinforced by countless examples offered by every group of living animals.  If such structures have not evolved naturally by degenerating from more efficient counterparts in ancestors of earlier times, and if they have been specially created, they are utterly meaningless and their very existence is unreasonable.  If common sense is to be employed, they demonstrate evolution.

Everywhere throughout the whole series animals place themselves in a treelike arrangement, for in their respective levels they occur like leaves at the ends of the lines of descent which have led up to them and which are comparable to the branches and limbs arising from the trunk of a tree.  Thus the major and minor divisions of animals do not follow in the order of the rungs of a ladder, even though they must be assigned to different levels according to the complexity of their construction.  The summary given above, namely, that the occurrence of lower and higher levels reveals an order of evolution, is amplified and not contradicted by the statement that the species of animals are group in a treelike arrangement.  It is the task of the evolutionist, provided with all the facts of comparative anatomy and dealing only with the various species as separate leaves, so to speak, to reconstruct the now invisible but not unreal twigs and branches and limbs of the animal tree, and to show how they have diverged at one time or another as they have grown and spread to produce the species of the present day.  This he may do in so far as he may find sufficient materials to enable him to employ the methods of comparative anatomy and the great natural principle established by this method—­that essential likeness means consanguinity.

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No evidence of evolution could be more significant and interesting than the results provided by the comparative study of development.  In the first place it is an obvious fact that every living thing changes in the course of its life-history, and if as an adult it occupies a high place in the animal scale, its embryological transformation is more elaborate and intricate than in the case of a lower form.  Every one knows that organisms do develop, and yet I believe that few appreciate the tremendous significance of the mere fact that this is true, while still fewer are aware that the peculiar and characteristic early stages through which an animal passes in becoming an adult are even more striking than the fact of development itself.  We shall learn something of these earlier conditions in the development of some of our most familiar animals, but at the outset nothing can be more important than an appreciation of the first great lesson of this department of natural history—­namely that organic transformation is real and natural.  We do not need to employ the methods of formal logic