88 THE ORIGIN OF SPECIES 



natural history. And I look at varieties which are in 

 any degree more distinct and permanent as steps toward 

 more strongly-marked and permanent varieties; and at 

 the latter as leading to sub-species, and then to species. 

 The passage from one stage of difference to another may, 

 in many cases, be the simple result of the nature of the 

 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 attributed 

 to the cumulative action of natural selection, hereafter 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 peri- 

 V ods, 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 f 

 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. t 



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 



