52 
VAEIETIES GRADUATE INTO SPECIES. Chap. IT. 
strongly marked and more permanent yarieties ; 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 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 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 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. 
