Evidence of Mans Descent

Embryological Evidence.

In recent years the formation of the human embryo in the womb, the complicated changes which transform the embryo into a foetus and the elaborate processes which produce the organs of the ripe child from embryonic rudi ments, have been studied by an ever-growing army of enquirers and by methods which possess an ever-increasing precision. Em bryologists find it necessary to assume that the law of evolution holds for man ; unless they make this assumption they can offer no rational explanation of the complex changes which engage their attention. In its broad lines development pursues the same course in the human body as in that of all vertebrate animals. What Francis Balfour in 1885 saw taking place with diagrammatic clear ness in the embryo of the dog-fish has given clues to the more complex and obscure processes now known to occur in the em bryo of man.

The developing human egg, when it becomes established in the mother's womb, undergoes a series of elaborate and peculiar changes. The investigations carried out by the late Dr. Emil Selenka (Menschenaffen. Studien iiber Entwickelung and Schlidel bau, 1898-1906) revealed the fact that only in the wombs of four other living mammals, the gorilla, chimpanzee, orang and gibbon, do the same changes take place. The process by which the placenta is formed, thus establishing a means of supplying the unborn child with nourishment, is exactly the same in man as in anthropoid apes. It is true that in Tarsius we see outlined the basal plan of placentation met with in the higher primates, but it is also true that in the placentation of the monkeys of the Old World and also in that of the New World we see a stage which leads on from the lower or tarsioid condition to the higher or anthropoid form. In the embryos of man and of the anthropoids an external jointed tail is produced in the fifth week of develop ment ; by the end of the eighth week it has shrivelled and be comes submerged, leaving a dimple at the point of the caudal region where it sinks below the surf ace. These are a few examples of some of the remarkable similarities which link the embryologi cal history of man with that of the anthropoid apes.

Recapitulation, Interpolatiori, Adaptation.

When in the later decades of the 19th century anatomists applied themselves to unravelling the development of man's body, they expected it to recapitulate, in full detail, the various stages of his evolution. In this they have been disappointed, because in the growth of the embryo and of the foetus we see three different processes at work. We see recapitulation taking place ; we also see new characters being interpolated from the time the embryo makes its first ap pearance until all the parts of a formed child are laid down; further we see at every stage the body of the embryo and of the foetus being adapted to a life within the womb. When gill-clefts appear in the neck of the human embryo towards the end of the first month of development we see a recapitulatory and very dis tant phase exemplified. When we look at the developing human foot, with the expectation of finding an anthropoid phase, we search in vain. The great toe is never a free and separate mem

ber in the human foot as in the adults of all other primates. There is a stage in the development of the feet of primates when all the digits diverge equally from the tarsal base ; man and ape pass through this stage and man clings to it as it were, whereas all the other primates pass on to a final prehensile stage. Yet in the sole of the newly born child we see the same flexion lines as in that of the gorilla ; we find the same muscles in the great toe of the human foot as in that of the gorilla ; we find the joint at the base of man's great toe, especially in the foetus, moulded in the same form as the gorilla. We cannot explain these appearances unless we believe that the human foot has been evolved from one like that of the gorilla, more especially as the foot of the gorilla shows a curious blend of human and monkey-like features.

The human great toe does not recapitulate ancestral history; developmental changes which mould the great toe into the human pattern set in just when the simian ones are due; the human changes do not succeed but replace those that give the ape its prehensile foot. New changes have been intercalated in the evolu tionary sequence. In a multitude of details the human embryo no longer recapitulates the series of changes gone through by its ancestors. It is true of every part of the human body; human characters begin to peer through its higher primate qualities be fore development is a month old.

Of the changes which affect the developing human body those which represent adaptations to life within the womb are the most important. The child draws its living from its mother's body; it is sheltered and kept warm ; it has not to seek its living nor de fend itself ; such qualities need not be attained until the time of birth ; until then nature is free to work out what experiments she will. It is a remarkable fact that many of man's distinguishing features are to be met with during foetal stages in the development of anthropoid apes. A stage which is transient in the foetal ape becomes permanent in man. We may take as an example the comparative hairlessness of man's body. A foetal chimpanzee, in the eighth month of development, resembles a human foetus of the same age; both have hair growing freely on their scalps, but the rest of their bodies, although provided with lanugo, appear to be nude. By birth the chimpanzee's body is covered with hair, but the human child retains the foetal state. Yet all known pri mates save man have their bodies thickly covered with hair; hairlessness is not an ancestral condition, but one made possible by the retention of the young in the shelter of the womb. The skin provides us with another example of foetal inheritance. In the fair or white stock of mankind the skin has become relatively free from pigment. In their earlier stages of foetal development apes are unpigmented ; they darken as the time of birth ap proaches. White men have come by their colouring through the inheritance of a foetal condition, one which is certainly not ancestral.