52 VARIETIES GRADUATE INTO SPECIES. Chap. If. 



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 

 justly called an incipient 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 becpme extinct, or they may 

 endure as varieties for very long periods, as has been 

 shown to be the case by Mr. Wollaston with the varie- 

 ties 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 spe- 

 cies 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. 



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 com- 

 parison with mere individual differences, is also applied 

 arbitrarily, and for mere convenience sake. 



