A problem in human evolution.

  • Allen, Grant.
Date:
[1879]
Catalogue details

Licence: Public Domain Mark

Credit: A problem in human evolution. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    A PROBLEM IN HUMAN EVOLUTION. The great key to the whole problem lies, it would seem, in die ict thus forced upon our attention, that the back of man forms le specially hairless region of his body. Hence we must conclude iat it is in all probability the first part which became entirely enuded of hair. Is there any analogy elsewhere which will enable s to explain the original loss of covering in this the normally airiest portion of the typical mammalian body . The eicct position ,f man appears immediately to suggest the requiiod analog} m the nost hairless region of other mammals. Almost all animals except man habitually lie upon the under sur- .ace of the body. Hence arises a conspicuous difference between the jack and the lower side. This difference is seen even in lizards, crocodiles, and other reptiles, amongst which, as a rule, the tegu- mentary modifications of the under surface are much less extended and less highly differentiated than those of the upper. It is seen amongst birds, which usually have the plumage far less copious on the breast than on the back. But it is most especially noticeable in ' mammals, which have frequently the under side almost entirely bare of hair, while the back is covered with a copious crop. Now, it would seem as though this scantiness of natural clothing on the under side were due to long-continued pressure against the ground, causing the hair to be worn away, and being hereditarily transmitted in its effects to descendants. * We are, therefore, led to inquire whether all parts of the mammalian body which come into frequent contact with other objects are specially liable to lose their hair. The answer seems to be an easy one. The soles 01 the feet in all mammals are quite hairless where they touch the ground, llie palms of the hands in the quadrumana presenu the same pheno- menon. The knees of those species which frequently kneel, such as camels and other ruminants, are apt to become bare and hard- skinned. The callosities of the old-world monkeys, which sit upon their haunches, are other cases in point; but they do not occur among- the more strictly arboreal quadrumana of the American con- tinent, nor among the lemurs, for the habits of these two classes in this respect are more similar to those of ordinary mammals. On the other hand, the new-world monkeys possess a prehensile tail, with which they frequently swing from bough to bough or lower them- selves to the ground, and in these creatures, says Cuvier, “la partie prenante de la queue est nue cn dessous.” Wherever we find a similar or^an, no matter how widelv different may be the structure and genealogy of the animals which possess it, we always find the prehensile portion free from hair. This is the case with the mar- supial tarsipes, with many rodents, and above all with the opossum, which uses its tail quite as much as any monkey uses its hands. Accordingly its surface is quite bare from end to end, and in some