Page - 261 - in The Origin of Species

THEORY OF NATURAL SELECTION 261 the wings of birds or bats, or the sudden conversion of a Hipparion into a horse, hardly any Hght is thrown by the be- lief in abrupt modifications on the deficiency of connecting links in our geological formations. But against the belief in such abrupt changes, embryology enters a strong protest. It is notorious that the wings of birds and bats, and the legs of horses or other quadrupeds, arc undistinguishable at an early embryonic period, and that they become differentiated by in- sensibly fine steps. Embryological resemblances of all kinds can be accounted for, as we shall hereafter see, by the pro- genitors of our existing species having varied after early youth, and having transmitted their newly acquired char- acters to their offspring, at a corresponding age. The em- bryo is thus left almost unaffected, and serves as a record of the past condition of the species. Hence it is that existing species during the early stages of their development so often resemble ancient and extinct forms belonging to the same class. On this view of the meaning of embryological resem- blances, and indeed on any view, it is incredible that an ani- mal should have undergone such momentous and abrupt trans- formations, as those above indicated ; and yet should not bear even a trace in its embryonic condition of any sudden modi- fication ; every detail in its structure being developed by in- sensibly fine steps. He who believes that some ancient form was transformed suddenly through an internal force or tendency into, for in- stance, one furnished with wings, will be almost compelled to assume, in opposition to all analogy, that many individuals varied simultaneously. It cannot be denied that such abrupt and great changes of structure are widely different from those which most species apparently have undergone. He will further be compelled to believe that many structures beautifully adapted to all the other parts of the same creature and to the surrounding conditions, have been suddenly pro- duced ; and of such complex and wonderful co-adaptations, he will not be able to assign a shadow of an explanation. He will be forced to admit that these great and sudden trans- formations have left no trace of their action on the embryo. To admit all this is, as it seems to me, to enter into the realms of miracle, and to leave those of Science.