Chap. II.] DOUBTFUL SPECIES. 65 



impresses the mind with the idea of an actual 

 passage. 



Hence I look at individual differences, though of 

 small interest to the systematise as of the highest 

 importance for us, as being the first steps towards such 

 slight varieties as are barely thought worth recording 

 in works on natural history. And I look at varieties 

 which are in any degree more distinct and permanent, 

 as steps towards 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 therefore be called an 

 incipient species ; but whether this belief 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 

 extinct, or they may endure as varieties for very long 

 periods, 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 



