vauiation under domestication. 7 



domesticated animals are still capable of rapid improve- 

 ment or modificationc 



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

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

 directly on the wliole organization or on certain parts 

 alone^ and indirectly by aifecting the reproductive 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 incidently shown in my work on 

 '^Variation under Domestication,^^ there are two factors: 

 namely; the nature of the organism and the nature of the 

 conditionSc The former seeuis to be much the more 

 important, for nearly similar variations sometimes arise 

 under, as far as we can judge, dissimihir conditions; aud, 

 on the other hand, dissimilar variations arise under condi- 

 tions 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 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 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 

 uniformly 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 

 outgrowths which variably follow from the insei'tiou of a 

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

 what singular modifications might result in the case ot 

 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 end- 

 less slight peculiarities which distinguish the individuals 

 of the same species, and which cannot be accounted for by 

 inheritance from either parent or from some more remote 



