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 different 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. 
