DIRECT ADAPTATION. 235 



itself, its reaction against the external influences, its change 

 by practice, habit, use, or non-use of organs, is put into the 

 foreground, then we forget that this reaction is first called 

 into play by the action of external conditions of existence. 

 Hence it seems that the distinction made between these two 

 groups lies only in the difl*erent manner of viewing them, 

 and I believe that they can, with full justice, be considered 

 as one. The most essential fact in these phenomena of 

 cumulative adaptation is that the change of the organism 

 which manifests itself first in the functions, and at a later 

 period in the form, is the result either of long enduring, or 

 of often repeated, influences of an external cause. The 

 smallest cause, by cumulation of its action, can attain the 

 greatest results. 



There are innumerable examples of this kind of direct 

 adaptation. In whatever direction we may examine the 

 life of animals and plants, we discover on all hands 

 evident and undeniable changes of this kind. Let me first 

 mention some of those phenomena of adaptation occasioned 

 directly by nutrition itself. Every one knows that the 

 domestic animals which are bred for certain purposes can 

 be variously modified, according to the different quantity 

 and quality of the food given to them. If a farmer in 

 breeding sheep wishes to produce fine wool, he gives them 

 different food from what he would give if he wished to obtain 

 good flesh or an abundance of fat. Choice race and 

 carriage horses receive better food than dray and cart 

 horses. Even the bodily form of man — for example, the 

 amount of fat — is quite different according to his nutrition. 

 Food containing much nitrogen produces little fat, that 

 containing little nitrogen produces a great deal' of fat. 



