8 VARIATION UNDER DOMESTICATION. [Chap. I. 



bability in the view propounded by Andrew Knight, 

 that this variability may be partly connected with 

 excess of food. It seems clear that organic beings 

 must be exposed during several generations to new 

 conditions to cause any great amount of variation ; and 

 that, when the organisation 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 do- 

 mesticated animals are still capable of rapid improv- 

 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 organisation or on certain 

 parts alone, and indirectly by affecting the reproductive 

 system. With respect to the direct action, we must 

 bear in mind that in every case, as Professor Weis- 

 mann has lately insisted, and as I have incidentally 

 shown in my work on ' Variation under Domestication,' 

 there are two factors : namely, the nature of the organ- 

 ism, and the nature of the conditions. The former seems 

 to be much the more important ; for nearly similar vari- 

 ations sometimes arise under, as far as we can judge, 

 dissimilar conditions ; and, on the other hand, dissimilar 

 variations arise under conditions which appear to be 

 nearly uniform. The effects on the offspring are either 

 definite or indefinite. They may be considered as defi- 

 nite when all or nearly all the offspring of individuals 

 exposed to certain 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 



