130 STERILITY FROM Chap. XVIII. 



chemical element deficient in the other and in sufficient 

 quantity to influence the whole after-growth of the plant. 

 As plants after once germinating are fixed to the same spot, 

 it might have heen anticipated that they would show the 

 good efiects of a change more plainly than do animals which 

 continually w^ander about ; and this aj^parently is the case. 

 Life depending on, or consisting in, an incessant play of the 

 most complex forces, it would appear that their action is 

 in some way stimulated by slight changes in the circum- 

 stances to which each organism is exposed. All forces through- 

 out nature, as Mr. Herbert Spencer ^ remarks, tend towards 

 an equilibrium, and for the life of each organism it is neces- 

 sary that this tendency should be checked. These views and 

 the foregoing facts probably throw light, on the one hand, 

 on the good efiects of crossing the breed, for the germ will 

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

 on the other hand, on the evil efiects of close interbreeding 

 prolonged 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 

 removed 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 con- 

 nection 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 favour- 

 able to plants and animals, and the crossing of varieties adds 



^ Mr. Spencer has fully and ably from cross-breeding, and of the evil 



discussed this whole subject in his effects from great changes in the con- 



' Principles of Biology,' 1864-, vol. ii. ditions and from crossing widely dis- 



ch. X. In the first edition of my tinct forms, as a series of facts " con- 



* Origin of Species,' 1859, p. 267, I nected together by some common but 



spoke of the good effects from slight unknown bond, which is essentially 



changes in the conditions of life and related to the principle of life. 



