290 Heredity. 



fications, and failing to ^nd any such proof, ho has ac- 

 cepted, as the only alternative, the view that variations 

 are fortuitous. This is not the only alternative, for 

 we see that there is a third view, namely, that, changed 

 conditions cause the variation, but do not determine its 

 character. 



In his exhaustive essav on Variation^ Darwin has dis- 

 cussed the question whether the external conditions of 

 life have such a direct and definite influence that the 

 exposure of many individuals for many generations to 

 any change in their physical conditions will result in the 

 modification of all or nearly all of them in the same 

 direction, thus producing a new sub-variety without 

 the aid of selection. 



He points out that many animals and plants which 

 range widely and are exposed to great diversity of con- 

 ditions remain nearlv the same in character: that the 

 two hundred plants which are distributed over every 

 English county, and which must have been exposed for 

 an immense period to considerable differences of climate 

 and soil, are uniform throughout the whole area; and 

 that certain birds, insects and plants which range over 

 large portions of the world, nevertheless retain the same 

 character. 



He calls attention to the fact that fowls and pigeons 

 have varied, and will no doubt go on varying, in directly 

 opposite ways, though kept during many generations 

 under nearly the same conditions; and he therefore con- 

 cludes that the amount of modification which animals 

 and plants have undergone under domestication does 

 not correspond with the degree to which they have been 

 exposed to changed circumstances. He lays especial 

 stress, in this connection, upon the phenomena of bud- 

 variation, and says: '*It is well worth while to reflect 



