26 ORIGIN OF SPECIES 



wheat, still yield new varieties : our oldest domesticated ani- 

 mals are still capable of rapid improvement 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 re- 

 spect 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 'Variation under Domesti- 

 cation,' there are two factors; namely, the nature of the 

 organism, and the nature of the conditions. The former 

 seems to be much the more important; for nearly similar 

 variations 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 definite 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 difificult to come to any con- 

 clusion 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, colour from the nature of the food, thick- 

 ness of the skin and hair from climate, etc. Each of the 

 endless variations which we see in the plumage of our fowls 

 must have had some efficient cause; and if the same cause 

 were to act uniforml}'^ during a long series of generations on 

 many individuals, all probably would be modified in the 

 same manner. Such facts as the complex and extraordinary 

 out-growths which variably follow from the insertion of a 

 minute drop of poison by a gall-producing insect, show us 

 what singular modifications might result in the case of plants 

 from a chemical change in the nature of the sap. 



Indefinite variability is a much more common result of 

 changed conditions than definite variability, and has prob- 

 ably played a more important part in the formation of our 

 domestic races. We see indefinite variability in the endless 

 slight peculiarities which distinguish the individuals of the 

 same species, and which cannot be accounted for by inher- 



