142 Heredity. 



trict to district, and their food and treatment varies 

 considerably. 



We accordingly find that, with few exceptions, all our 

 domesticated animals and plants vary more than their 

 wild relations. Even the goose, one of the least varia- 

 ble of domesticated animals, varies more than almost 

 any wild bird, and according to Darwin, hardly a single 

 plant can be named, which has long been propagated 

 and cultivated by seed, that is not highly variable. 



These considerations force us to conclude that varia- 

 bility is not a necessary contingent of reproduction, but 

 that the production of the gemmules which give rise to 

 variation is excited by changes in external conditions, 

 and we must agree with Darwin that " it is probable 

 that variability of every kind is directly or indirectly 

 caused by changed conditions of life; or to put the case 

 under another point of view, if it were possible to ex- 

 pose all the individuals of a species during many gener- 

 ations to absolutely uniform conditions of life, there 

 would be no variability." 



When we come to examine the effect of different con- 

 ditions of life we find that we cannot attribute the varia- 

 bility to one rather than the other. The essential thing 

 is change, but not any particular kind of change. 



Variation is frequently caused by a change of climate, 

 but this is by no means essential, for most cultivated 

 plants yield more varieties when cultivated in their na- 

 tive country than when removed to other climates. 

 (Darwin, Variation, ii. p. 310.) 



Change of food is often a cause of variation, but that 

 this is not necessary is shown by the fact pointed out by 

 Darwin, that fowls and pigeons are the most variable of 

 domesticated animals, although their food is nearly the 

 game as that of their wild allies, but is much less varied 



