Fig. 2. Showing how the great arteries of the thorax and neck are derived from the arteries of the gill-arches.
The developmental changes which have just been described relate to remote stages in the evolution of the human body ; those now to be described seem to belong to a later stage. When the hinder end of the body of a human foetus is examined during the second month of development, it is seen that the bowel, the genital and urinary ducts end in a common chamber, or cloaca. By the end of the third month certain changes have occurred, which separate the opening of the bowel from the uro-genital passage. Occasionally children are born in which that separation has failed to occur, so that there is present only the primitive passage. When this condition is contrasted with the arrangement of parts seen in vertebrate animals, we find that it is similar to the condition present in fishes, amphibia, reptiles, birds and in the very lowest mammals in which there is only one passage—the cloaca. In all the higher mammals there are two passages, and we infer that the division of the posterior vent must have occurred when the marsupial stock branched off from the ancestral line of the higher mammals and therefore at a date long prior to the appearance of the human stock. In the subjects of the unfortunate malformation just described, we see a reversion to a stage of evolution much less remote than those maldevelopments connected with the imperfect heart and branchial arches.
The instances we have so far cited from the development of the human body indicate in only a general way certain evolutionary phases which are long past. We are now to see if there are any malformations which indicate for us one of the later phases of human evolution—especially such a stage as we may presume to have existed when the human and anthropoid stocks diverged. There are two parts of the body which may be expected to supply us with evidence. These are the essentially human structures—the foot and the brain. So far as concerns the foot, it may be said definitely that a child has never been seen with an anthropoid foot. Yet there is one form of congenital malformation of the foot—congenital club foot—which has certain resemblances to the ape's foot. It is like the ape's, in that the feet are turned inwards, so that the soles face each other; when the child walks it places the outer border of the foot on the ground as the ape does. But the great toe differs from that of the ape ; it is not set on the sole as the thumb is in the hand. Yet in the sole of the foot of even the normal child there may be seen the same fold which is present in the ape's foot between the root or ball of the great toe and the rest of the sole. In Eastern races the great toe can be used for grasping objects.
The brain may be arrested in its development. The best examples are to be found in small-headed or microcephalic idiots. There are many cases on record, but the one I propose to bring before the reader is the brain of " Joe," which was investigated by the late Professor Cunningham—one of the best anatomists of our time. Joe's brain weight was 560 grammes, 900 less than that of a normal man and only slightly heavier than that of a gorilla. He was 5 ft. 9 in. high and died at the age of sixty. His head was very small, especially in comparison with his face, which was of normal proportions. He spent the latter part of his life in the county asylum of Lancashire, where he tended the sheep, keeping them within " prescribed limits " with great vigilance for days together. He had command of a considerable number of words and could frame sentences. The expression of his face " was distinctly human," and although he had a vacant imbecile look, his face clearly reflected every emotion in a perfectly human fashion. He knew a sixpence from a fourpenny bit; he could count his fingers, but did not know much about the divisions of time; he was easily puzzled, and did not know how many years there were in a week.
When " Joe " died, his brain was found to be not only small in size, but to have its fissures and convolutions arranged in a very simple manner. Professor Cunningham was an extremely judicious and cautious observer. The features of " Joe's " brain could not be explained by supposing it had simply been arrested at a certain stage of development, for although the fissures and convolutions had resemblance to the condition seen in the fcetal brain in the seventh month, yet there were other additional and modifying features which he was inclined to regard as the reproduction —an imperfect reproduction perhaps—of a stage in the evolution of the human brain. Certain appearances were really anthropoid in nature, yet in complexity of pattern " Joe's " brain was simpler than that of a chimpanzee. The posterior or occipital part of the cerebrum was particularly small; we know that it is just this part which forms a relatively large part of the brain of the primates. The fissure of Sylvius was placed obliquely, and that area of the brain—the Island of Reil—which is submerged and hid in the fissure in the normal human brain, was exposed and its front part was continuous with the inferior frontal convolution as in anthropoids. The parts of the brain connected with speech were as small as in the anthropoid ape. It seems possible then that such a brain as that of Joe" does represent, in a disturbed and somewhat distorted manner, an actual stage in the evolution of the human brain.
The most remarkable instance of the close relationship between the anthropoids and man is to be found in their earliest stages of development. It is only within the last fifteen years that we have got to know these stages. The late Professor Emil Selenka spent his fortune and his leisure in procuring and in investigating the embryos of anthropoid apes, and several observers, both at home and abroad, have published exact accounts of human embryos under fourteen days old during quite recent years. Not only is the uterus of the anthropoid and of mankind similar in form, but their embryos become implanted in an exactly similar manner, a manner which is only known to occur in them.