184 THE DESCENT OP MAN. 



bined with those other causes which tend to check the increase 

 of every population, would sooner or later lead to extinction. 

 The diminution of fertility may be explained in some cases by 

 the profligacy of the women (as until lately with the Tahitians) 

 but Mr. Fenton has shown that this explanation by no means suf- 

 fices with the New Zealanders, nor does it with the Tasmanians. 



In the paper above quoted, Mr. Macnamara gives reasons for 

 believing that the inhabitants of districts subject to malaria are 

 apt to be sterile; but this cannot apply in several of the above 

 cases. Some writers have suggested that the aborigines of is- 

 lands have suffered in fertility and health from long continued 

 inter-breeding; but in the above cases infertility has coincided 

 too closely with the arrival of Europeans for us to admit this 

 explanation. Nor have we at present any reason to believe 

 that man is highly sensitive to the evil effects of inter-breeding, 

 especially in areas so large as New Zealand, and the Sandwich 

 archipelago with its diversified stations. On the contrary, it is 

 known that the present inhabitants of Norfolk Island are nearly 

 all cousins or near relations, as are the Todas in India, and the 

 inhabitants of some of the Western Islands of Scotland; and yet 

 they seem not to have suffered in fertility.** 



A much more probable view is suggested by the analogy of the 

 lower animals. The reproductive system can be shown to be sus- 

 ceptible to an extraordinary degree (though why we know not) 

 to changed conditions of life; and this susceptibility leads both 

 to beneficial and to evil results. A large collection of facts on 

 this subject is given in chap, xviii. of vol. ii. of my 'Variation of 

 Animals and Plants under Domestication,' I can here give only 

 the briefest abstract; and every one interested in the subject may 

 consult the above work. Very slight changes increase the health, 

 vigor and fertility of most or all organic beings, whilst other 

 changes are known to render a large number of animals sterile. 

 One of the most familiar cases, is that of tamed elephants, not 

 breeding in India; though they often breed in Ava, where the 

 females are allowed to roam about the forests to some extent, and 

 are thus placed under more natural conditions. The case of 

 various American monkeys, both sexes of which have been kept 

 for many years together in their own countries, and yet have 

 very rarely or never bred, is a more apposite instance, because of 

 their relationship to man. It is remarkable how slight a change 

 in the conditions often induces sterility in a wil'd animal when 

 eaptured; and this is the more strange as all our domesticated 



" On the close relationship of the Norfolk Islanders, see Sir W. 

 Denison, 'Varieties of Vice-Regal Life,' vol. i. 1870, p. 410. For the 

 Todas, see Col. Marshall's work, 1873, p. 110. For the Western Islands 

 of Scotland, Dr. Mitchell, 'Edinburgh Medical Journal,' March to 

 June, 1865. 



