148 



STERILITY FROM 



Chap. XVIII. 



case. Life depending on, or consisting in, an incessant play 



plex forces, it would appear that 



action is in 



some way stimulated by slight changes in the circumstances t( 

 which each organism is exposed. All forces throughout nature 

 as Mr. Herbert Spencer 8 remarks, tend towards an equili 

 brium, and for the life of each being it is necessary that thii 



tendency should be checked. 



facts can be trusted, they probably throw light, on the 



If these views and the foreo- 



on the good effects of crossing the breed, for the germ will be 

 thus slightly modified or acted on by new forces ; and on the 



hand, on the evil effects of close interbreeding prolon 



to 



during many generations, during which the germ will be acted 

 on by a male having almost identically the same constitution. 



Sterility from changed Conditions of Life. 



* 



I will now attempt to show that animals and plants, when re- 

 moved from their natural conditions, are often rendered in some 

 degree infertile or completely barren ; and this occurs even when 

 the conditions have not been greatly changed. This conclusion 

 is not necessarily opposed to that at which we have just arrived, 

 namely, that lesser changes of other kinds are advantageous to 

 organic beings. Our present subject is of some importance, from 

 having an intimate connexion with the causes of variability. 

 Indirectly it perhaps bears on the sterility of species when 

 crossed : for as, on the one hand, slight changes in the conditions 

 of life are favourable to plants and animals, and the crossing of 

 varieties adds to the size, vigour, and fertility of their offspring ; 



other hand, certain other changes in the conditions 



so, on the other 



of life cause sterility 



; and as this likewise ensues from 

 crossing much-modified forms or species, w r e have a parallel 

 and double series of facts, which apparently stand in close rela- 

 tion to each other. 



It is notorious that many animals, though perfectly tamed 



} 



8 Mr. Spencer has fully and ably dis- 



breeding, and of the evil effects from 



J " TXX ' NJJ^VX±VV7J. j^v*.^ *^**j ^^.^ u/^xj ^"^ ^lUULUliQ, WAXK+ Vi v* ^ ^ - 



cussed this whole subject in his ' Prin- great changes in the conditions an 



ciples of Biology,' 1864, vol. ii. ch. x. from crossing widely distinct forms, as 



In the first edition of my ' Origin of a series of facts "connected together by 



some common but unknown bond, wtnc i 



is essentially related to the principle oi 

 life." 



Species,' 1859, p. 267, I spoke of the 

 good effects from slight changes in 

 the conditions of life and from cross- 



