44 The Deszent of Man. Paht L 



part. Various monstrosities, as I. Geoffroy repeatedly insists, are 

 thus intimately connected. Homologous structures are par- 

 ticularly liable to change together, as we see on the opposite 

 Bides of the body, and in the upper and lower extremities. 

 Meckel long ago remarked, that when the muscles of the arm 

 depart from their proper type, they almost always imitate those 

 of the leg ; and so, conversely, with the muscles of the legs. The 

 organs of sight and hearing, the teeth and hair, the colour of the 

 skin and of the hair, colour and constitution, are more or less cor- 

 related.^ Professor Schaaffhausen first drew attention to the 

 relation apparently existing between a muscular frame and 

 the strongly-pronounced supra-orbital ridges, which are so 

 characteristic of the lower races of man. 



Besides the variations which can be grouped with more oi 

 loss probability under the foregoing heads, there is a large class 

 of variations which may be provisionally called spontaneous, for 

 to our ignorance they appear to arise without any exciting 

 cause. It can, however, be shewn that such variations, whether 

 consisting of slight individual differences, or of strongly-marked 

 and abrupt deviations of structure, depend much more on the 

 constitution of the organism than on the nature of the condi- 

 tions to which it has been subjected.'* 



Bate of Increase. — Civilised populations have been known 

 under favourable conditions, as in the United States, to double 

 their numbers in twenty-five years ; and, according to a calcula- 

 tion by Euler, this might occur in a little over twelve years.*' At 

 the former rate, the present population of the United States 

 (thirty millions), would in 657 years cover the whole terraqueous 

 globe so thickly, that four men would have to stand on each 

 Bquare 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. \\'ith civilised nations this primary check 

 acts chiefly by restraining marriages. The greater death-rate of 

 infants in the poorest classes is also very important; as well as 



•' The authorities for these seve- my 'Variation of Animals and Planle 



ral statements are given in my under Domestication.* 

 * Variation of Animals under Do- ^' See the ever memorable * Essay 



nesticaticQ,' vol. ii. pp. 320-335. on the Principle of Population,' bj 



*' This whole snbject hiis been the Rev. T. Malthus, vol. i. 1826, p 



4l3(^asi:'e I m chaf . xxiil. vol. ii. ot 6. 517. 



