From the natural history of the anthropoid apes, Huxley passed to consideration of their relation to man, prefacing his observations with a passage defending the utility of the enquiry, a passage necessary enough in these days of prejudice, but now chiefly with historical interest: 

“It will be admitted that some knowledge of man’s position in the animate world is an indispensable preliminary to the proper understanding of his relations to the universe; and this again resolves itself in the long run into an enquiry into the nature and the closeness of the ties which connect him with those singular creatures whose history has been sketched in the preceding pages.

      “The importance of such an enquiry is, indeed, intuitively
     manifest.  Brought face to face with these blurred copies of
     himself, the least thoughtful of men is conscious of a certain
     shock; due perhaps not so much to disgust at the aspect of what
     looks like an insulting caricature, as to the awakening of a
     sudden and profound mistrust of time-honoured theories and
     strongly rooted prejudices regarding his own position in nature,
     and his relations to the underworld of life; while that which
     remains a dim suspicion for the unthinking, becomes a vast
     argument, fraught with the deepest consequences, for all who are
     acquainted with the recent progress of the anatomical and
     physiological sciences.”

Huxley then proceeded to elaborate the argument from development for the essential identity of man and the apes.  This argument has now become more or less familiar to us all, as it has gained additional support from recent extension of embryological knowledge, and as it has been used in every work on evolution since Huxley first laid stress on it.  The adult forms of animals are much more complex than their embryonic stages, and the series of changes passed through in attaining the adult condition make up the embryological history of the animal.  Huxley took the embryology of the dog as an example of the process in the higher animals generally, and as it had been worked out in detail by a set of investigators.  The dog, like all vertebrate animals, begins its existence as an egg; and this body is just as much an egg as that of a fowl, although, in the case of the dog, there is not the accumulation of nutritive material which bloats the egg of the hen into its enormous size.  Since Huxley wrote, it has been shewn clearly that among the mammalian animals there has been a gradual reduction in the size of the egg.  The ancestors of the mammals laid large eggs, like those of birds or reptiles; and there still exist two strange mammalian creatures, the Ornithorhynchus and Echidna of Australia, which lay large, reptilian-like eggs.  The ancestors of most living mammalia acquired the habit of retaining the eggs within the body until they were hatched; and, as a result of this, certain structures which