48 ON THE ORIGIN OF SPECIES 



come very near to, but do not quite arrive at the 

 rank of species ; or, again, between sub-species and 

 well-marked varieties, or between lesser varieties and 

 individual differences. These differences blend into 

 each other in an insensible series ; and a series im- 

 presses the mind with the idea of an actual passage. 



Hence I look at individual differences, though of 

 small interest to the systematist, as of high importance 

 for us, as being the first step 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 

 leading to more strongly marked and more permanent 

 varieties ; and at these latter, as leading to sub-species, 

 and to species. The passage from one stage of difference 

 to another and higher stage may be, in some cases, 

 due merely to the long-continued action of different 

 physical conditions in two different regions ; but I 

 have not much faith in this view ; and I attribute the 

 passage of a variety, from a state in which it differs 

 very slightly from its parent to one in which it differs 

 more, to the action of natural selection in accumulating 

 (as will hereafter be more fully explained) differences 

 of structure in certain definite directions. Hence I 

 believe a well-marked variety may be called an in- 

 cipient species ; but whether this belief be justifiable 

 must be judged of by the general weight of the several 

 facts and views given throughout this work. 



It need not be supposed that all varieties or incipient 

 species necessarily attain the rank of species. They 

 may whilst in this incipient state 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. 

 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 co-exist, and both rank as independent species. 

 But we shall hereafter have to return to this subject. 



