330 Darwin, and after Darwin. 



the same variety are exposed during several generations to 

 any change in their physical conditions of life, all, or nearly 

 all, the individuals are modified in the same manner. A new 

 sub-variety would thus be produced without the aid of selec- 

 tion^ 



As an example of the special instances that he gives, 

 I may quote the following from the same work : 



" 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 uniformlv during a long series of 

 generations on many individuals, all probably would be modi- 

 fied in the same manner." 



And, as instances of his more general statements in Chapter 

 XXIII, these may suffice : 



" The direct action of the conditions of life, whether leading 

 to definite or indefinite results, is a totally distinct consider- 

 ation from the effects of natural selection The 



direct and definite action of changed conditions, in contra- 

 distinction to the accumulation of indefinite variations, seems 

 to me so important that I will give a large additional body 

 of miscellaneous facts V 



Then, after giving these facts, and showing how in the 

 case of species in a state of nature it is often impossible to 

 decide how much we are to attribute to natural selection and 

 how much to the definite action of changed conditions, he 

 begins his general summary of the chapter thus: 



" There can be no doubt, from the facts given in the early 

 part of this chapter, that extremely slight changes in the 

 conditions of life sometimes act in a definite manner on our 

 already variable domesticated productions [productions, there- 

 fore, with regard to which uniiormity and "stability" of 

 modification are least likely to arise] ; and, as the action 

 of changed conditions in causing general or indefinite vari- 



1 Variation, &c., vol. ii.p. 260. a Ibid, vol.ii. p. 261. 



