64 ON CEOSSING AS A CAUSE. Chap. XV. 



this will lead to the mongrels increasing more rapidly than 

 the pure parent-breeds. 



When distinct breeds are allowed to cross freely, the result 

 will be a heterogeneous body ; for instance, the dogs in Para- 

 guay are far from uniform, and can no longer be affiliated to 

 their parent -races. 4 The character which a crossed body of 

 animals will ultimately assume must depend on several con- 

 tingencies, — namely, on the relative members of the individuals 

 belonging to the two or more races which are allowed to 

 mingle ; on the prepotency of one race over the other in the 

 transmission of character; and on the conditions of life to 

 which they are exposed. When two commingled breeds exist 

 at first in nearly equal numbers, the whole will sooner or 

 later become intimately blended, but not so soon, both breeds 

 being equally favoured in all respects, as might have been 

 expected. The following calculation 5 shows that this is the 

 case : if a colony with an equal number of black and white 

 men were founded, and we assume that they marry indis- 

 criminately, are equally prolific, and that one in thirty 

 annually dies and is born ; then "in 65 years the number of 

 " blacks, whites, and mulattoes would be equal. In 91 years 

 " the whites would be l-10th, the blacks l-10th, and the 

 " mulattoes, or people of intermediate degrees of colour, 

 " 8-10ths of the whole number. In three centuries not 

 " l-100th part of the whites would exist." 



When one of two mingled races exceed the other greatly 

 in number, the latter will soon be wholly, or almost wholly, 

 absorbed and lost, 6 Thus European pigs and dogs have been 

 largely introduced in the islands of the Pacific Ocean, and 

 the native races have been absorbed and lost in the course of 

 about fifty or sixty years ; 7 but the imported races no doubt 

 were favoured. Eats may be considered as semi-domesticated 

 animals. Some snake-rats (Mus alexandrinus) escaped in the 

 Zoological Gardens of London, " and for a long time after- 



4 Rengger, ' Saugethiere,' &c, s. maines,' p. 24, first called attention 

 154. to this subject, and ably discussed it. 



5 White, ' Regular Gradation in ~ Rev. D. Tyerman and Bennett, 

 Man,' p. 146. ' Journal of Voyages,' 1821-1829, vol. 



6 Dr. W. F. Edwards, in his ' Ca- i. p. 300. 

 racteres Physiolog. des Races Hu- 





