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THE AMERICAN NATURALIST [Vol. XLIII 



By the theory of natural selection all living species have been con- 

 nected with the parent species of each genus, by differences not greater 

 than we see between the natural and domestic varieties of the same 

 species at the present day; and those parent species now generally 

 extinct, have in their turn been similarly connected with more ancient 

 forms; and so on backwards always converging to the common ancestor 

 of each great class (p. 266). 



Here is a sentence which anticipates some recent specu- 

 lations respecting a so-called plastic period in the life of 

 a species : 



It is a more important consideration, leading to the same result, 

 as lately insisted on by Dr. Falconer, namely, that the period during 

 which each species underwent modification, though long as measured 

 by years, was probably short in comparison with that during which it 

 remained without undergoing change (p. 279). 



To the same purport is the following : 



This gradual increase in number of the species of a group is strictly 

 conformable with the theory, for the species of the same genus and 

 the genera of the same family, can increase only slowly and progress- 

 ively . . . one species first giving rise to two or three varieties, these 

 being slowly converted into species, which in turn produce by equally 

 slow steps other varieties and species ... (p. 293). 



Concerning the origin of varieties he has this to say: 



The complex and little known laws governing the production of 



have governed the production of distinct species (p. 415). 



In the following passage provision appears to be made 

 for the origin of varieties by mutation : 



I have now recapitulated the facts and considerations which have 

 thoroughly convinced me that species have been modified, during a 

 long course of descent. This has been effected chiefly through natural 

 selection of numerous successive, slight favorable variations; aided 

 in an important manner by the inherited effects of the use and disuse 

 of parts, and in an important manner, that is in relation to adaptive 

 structure, whether past or present, by the direct action of external 



spontaneously It appears that I formerly underrated the frequency 

 and value of these latter forms of variation, as leading to permanent 

 modifications of structure independently of natural selection, . . • 



He adds, in answer to certain recent criticisms that he 

 placed at the end of the introduction to his first edition, 

 the following words: 



