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Show 150 THE DESCENT OF 1\IAN. PART I. surprising that man should differ so greatly in hairiness .from all his lower brethren, for characters gained through sexual selection often differ in closely-related forms d!o an extraordinary degree. AICcordmg to a popular impression, the absence of a taH is eminently distinctive of man; but as those apes \vihich come nearest to man are destitute of this organ, Hs disappearance does not especially concern us. Nevertth. eless it may be well to own that no explanation, as far as I am .aware, has ever been given of the loss of the tai[ by certain apes and man. Its loss, however, is not surprising, for it sometimes differs remarkably in length in species of the same genera: thus in some species of Macacus the tail is longer than the whole body, consisting of twenty-four vertebrre; in others it consists of a scarcely visible stump, containing only three or four ve!rtebrre. In some kinds of baboons there are twentyfive, whilst in the mandrill there are ten very small stunted caudal vertebrre, or, accordinO' to Cuvier 79 sometimes oniJ.y five. This great diversity i~ the structure and length of t~e tail in animals belonging to the same genera, and followmg nearly the same habits of life, renders it probable that the tail is not of much importance to them~ and if so, we might have expected that it would. sometimes h~ve become more or less rudimentary, in accovdance With what we incessantly see with other structures. The tail almost always tapers towards the end whether it be long or short; and this, I presume, results from the atrophy, through disuse, of the terminal muscles together with their arteries and nerves leading to tbe atrophy of the terminal bones. With 1~spect 79 Mr. St. Geo~·ge Mivart, 'Proc. Zoolog. Soc.' 1865, p. 562, 583. ~r. J .. E. ~ray, .~at. Brit. l\1us.: Skeletons.' Owen, 'Anatomy of ~.e;~~~:es, vol. u. p. 517. Isidore Geoffroy, 'Hist. Nat. Gen.' tom. •CHAP. IV. 1\lANNER OF DEVELOPMENT. 151 to the os coccyx, which in man and the higher apes manifestly consists of the few basal and tapering segments of an ordinary tail, I have heard it asked how could these have become completely embedded within the body; but there is no difficulty in this respect, for in many monkeys the basal segments of the true tail are thus embedded. For instance, Mr. Murie informs me that in the skeleton of a not full-grown JJiacacus inornatus, he counted nine or ten caudal vertebrre, which altogether were only 1·8 inch in length. Of these the three basal ones appeared to have been embedded; the remainder forming the free part of the tail, which was only one inch in length, and half an inch in diameter. Here, then, the three embedded ·Caudal vertebrre plainly correspond with the :four coalesced vertebrre of the human os coccyx. I have now endeavoured to shew that some of the most distinctive characters of man have in all probability been acquired, either directly, or more commonly indirectly, through natural selection. We should bear in mind that modifications in structure or constitution, which are of no service to an organism in adapting it to its habits of life, to the food which it con. sumes, or passively to the surrounding conditions, cannot have been thus acquired. We must not, however, be too confident in deciding what modifications are of .service to each being : we should remember how little we know about the use of many parts, or what changes in the blood or tissues may serve to fit an organism for a new climate or some new kind of food. Nor must we forget the principle of correlation, by which, as Isidore Geoffroy has shewn in the case of man, many .strange deviations of structure are tied together. Inde. Pendently of correlation, a change in one part often leads |
