'S2 THE ORIGIN OF SFECIES 



generations to new conditions to cause any great amount 

 of variation; and that, when the organization has once 

 begun to vary, it generally continues varying for many 

 generations. No case is on record of a variable organism 

 ceasing to vary under cultivation. Our oldest cultivated 

 plants, such as wheat, still yield new varieties: our oldest 



L domesticated animals are still capable of rapid improve- 

 ment or modification. 



As far as I am able to judge, after long attending to 



the subject, the conditions of life appear to act in two 



,) ways — directly on the whole organization or on certain 



parts alone, and indirectly by affecting the reproductive 



/"t- system. "With respect to the direct action, we must bear 

 in mind that in every case, as Professor Weismann has 

 lately insisted, and as I have incidentally shown in my 

 work on "V^ariation under Domestication," there are two 

 factors; namel}'^, the nature of the organism, and the 

 nature of the conditions. The former seems to be much 

 the more important; for nearly similar variations some- 

 times arise under, as far as we can judge, dissimilar con- 

 ditions; and, on the other hand, dissimilar variations 

 arise under conditions which appear to be nearly uni- 

 form. The effects on the offspring are either definite or 

 indefinite. They may be considered as definite when all 

 or nearly all the offspring of individuals exposed to cer- 

 tain conditions during several generations are modified in 

 the same manner. It is extremely difficult to come to 

 any conclusion in regard to the extent of the changes 

 which have been thus definitely induced. There can, 

 however, be little doubt about many slight changes — 

 such as size from the amount of food, color from the 

 nature of the food, thickness of the skin and hair from 



