292 CAUSES OF VARIATION 



and it may be fairly spoken of as a primary cause of variation in 

 type. This does not make it, however, like temperature or food 

 supply, a fundamental cause of variation in living matter. There 

 is no basis for selection until differences have appeared from 

 other causes. When the selection is made, based on these differ- 

 ences, it is certain to affect the type, and it is therefore a cause 

 of type variation ; but it was no cause of the original differences 

 on which its action was predicated, and it is in no sense an 

 original cause of variability. The confusion of mind which causes 

 selection to be regarded as a cause of variation has arisen from 

 a failure to distinguish clearly between the individual and the 

 type, between the material on which we can work through 

 selection on the one hand, and the finished product on the other. 



Variation in type through the inheritance of modifications due 

 to environment. Whether individual modifications due to environ- 

 ment (acquired characters) are inherited will be discussed in a 

 following chapter. It is the purpose here only to show the nature 

 of the effect of such inheritance in case it does occur. 



If the modifications due to external influences should per- 

 chance be inherited, even in the slightest degree, then the effects 

 of modification would, like those of selection, become cumulative, 

 and the type would the more rapidly conform to the environment 

 and the more rapidly establish a " fit " with the conditions of life. 



It is evident that this is a difficult field. Merely through the 

 exigencies of maintenance those characters will develop best that 

 are most favored by the conditions of life, thus bringing about 

 a kind of mass response to the exactions of the environment. 

 Again, by the principle of selection, only those in fair accord with 

 the environment will live, and this brings about a still closer fit. 

 If, now, modifications due to environment were fully and com- 

 pletely inherited, the adjustment to constant conditions would 

 speedily become so exact and complete as to leave no. room for 

 selection. 



Few biologists are bold enough to claim this extreme degree 

 of inheritance of modifications due to environment. Many deny 

 it in toto. That neither extreme is right is comparatively easy 

 of as good proof as we are generally able to bring in affairs bio- 

 logical, but where between lies the truth it is most difficult to 



