78 DARWINISM STATED BY DARWIN HIMSELF. 



organism, and of the different physical conditions to 

 ■which it has long been exposed ; but with respect to the 

 more important and adaptive characters, the passage from 

 one stage of difference to another may be safely attrib- 

 uted to the cumulative action of natural selection, here- 

 after to be explained, and to the effects of the increased 

 use or disuse of parts. A well-marked variety may there- 

 fore be called an incipient species ; but whether this be- 

 lief is justifiable must be judged by the weight of the 

 various facts and considerations to be given throughout 

 this work. 



It need not be supposed that all varieties or incipient 

 species attain the rank of species. They may become ex- 

 tinct, or they may endure as varieties for very long pe- 

 riods, as has been shown to be the case by Mr. Wollaston 

 with the varieties of certain fossil land-shells in Madeira, 

 and with plants by Gaston de Saporta. If a variety were 

 to flourish so as to exceed in numbers the parent species, 

 it would then rank as the species, and the species as the 

 variety ; or it might come to supplant and exterminate 

 the parent species ; or both might coexist, and both rank 

 as independent species. But we shall hereafter return to 

 this subject. 



From these remarks it will be seen that I look at the 

 term species as one arbitrarily given, for the sake of con- 

 venience, to a set of individuals closely resembling each 

 other, and that it does not essentially differ from the 

 term variety, which is given to less distinct and more 

 fluctuating forms. The term variety, again, in compari- 

 son with mere individual differences, is also applied arbi- 

 trarily, for convenience' sake. 



