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Show 130 THE DESCENT OF MAN. PA.UT I. to those ·of rudimentary organs given in the first chapter, .that many of them might have been indifferently introduced in either chapter. Thus a human uterus furnished with cornua may be said to represent in a rudimentary condition the same organ in its normal state in certain mammals. Some parts which are rudimental in man, as the os coccyx in both sexes and the mammre in the male sex, are always present; whilst others, such as the supracondyloid foramen, only occasionally appear, and therefore might have been introduced under the head of reversion. These several reversionary, as well as the strictly rudimentary, structures reveal the descent of man from some lower form in an unmistakeable manner. Correlated Variation.-In man, as in the lower animals, many structures are so intimately related, that when ·one part varies so does another, without our bein()' ab[e, in most cases, to assign any reason. We cannot sa; whether the one part governs the other, or whether both are governed by some earlier developed part. Various monstrosities, as I. Geoffroy repeatedly insists, are thus intimately connected. Homologous structures are particulwrly liable to change together, as we see on the opposit~ ~ides of the body, and in the upper and lower extrem1ties. Meckel long ago remarked that when the muscles of th~ a:m depart from their proper type, they al:nost always Imitate those of the leg; and so conversely wrth. the muscles of the legs. The organs of sight and hearmg, the teeth and hair, the colour of the skin and 'hair, colour and constitution, are more or less correlated.49 Professor .Schaaffhausen first drew attention to the rela- , 40 :r'l~e authori~ics for these several statements o.re given in my Varmt10n of Ammals under Domestication,' vol. ii, p. 320-335. <CHAP. IV. MANNER OF DEVELOPMENT. 131 tion apparently existing between a muscular frame and strongly-pronounced supra-orbital ridges, which are so characteristic of the lower races of man. Besides the variations which can be grouped with more or less probability under the foregoing heads, there is a large class of variations which may be provisionally called spontaneous, for they appear, owing to our ignorance, to arise without any exciting cause. It can, however, be shewn that such variations, whether consisting of slight individual differences, or of stronglymarked and abrupt deviations of structure, depend much more on the constitution of the organism than <>n the nature of the conditions to which it has been subjected. 5° Rate of Increase.- Civilised populations have been known under favourable conditions, as in the United States, to double their number in twenty-five years; and according to a calculation by Euler, this might occur in a little over twelve years. 5 1 At the former rate the present population of the United States, namely, thirty millions, would in 657 years cover the whole -terraqueous globe so thickly, that four men would have to stand on each square yard of surface. The primary or fundamental check to the continued increase of man is the difficulty of gaining subsistence and of living in -comfort. We may infer that this is the case from what we see, for instance, in the United States, where subsistence is easy and there is plenty of room. If such means were suddenly doubled in Great Britain, our number would be quickly doubled. With civilised nations the 50 This whole subject has been discussed in chap. xxiii. vol. ii. of .my' Variation of Animals and Plants under Domestication: 51 See the ever memorable ' Essay on the Principle of Population,' ~y the Rev. T. Malthus, vol. i. 1826, p. 6~ 517. K 2 |
