Fig. 15.—Stages in development of human embryo.
a, 18-21 days; b, 27-30 days; c, 35 days; d, 52-54 days old.
The young flat-fish is like an ordinary member of the fish tribe, with an eye on either side of its head, and its body built on the ordinary symmetrical lines. It is only later, when it begins habitually to be upon one side on the sea bottom, that the eye from the under side wanders round to the opposite aspect beside its fellow, and the upper side becomes pigmented, while the lower remains white.
In similar fashion a primitive form of kidney is, as it were, sketched in, in the development of the higher animals, only to be erased at a later stage and replaced by a better form. The human child has a complete body covering of hair, which disappears soon after birth. In these and many more instances, one cannot avoid the impression that the organism has not been specially designed for what it finally comes to be. It cannot forget, and must needs repeat, or so it seems, some considerable part of the history of its race.
Manifestly, then, all this evidence, gleaned from many different sources, points to a common origin of living things, and to the gradual evolution of the higher from the lower types. It may also be said that there is no scientific evidence against such a view.
CHAPTER II
UNICELLULAR AND MULTICELLULAR ANIMALS
We must now turn to the main project of this book, which is to attempt to trace out the lines along which animal Evolution has proceeded, with special reference to that particular line which leads up to man. Indeed, we shall have to stick somewhat closely to this one main highway, and can but barely pause to glance along the numerous branch roads, interesting though the travelling there might be.