DOMINANT SPECIES VARY MOST. 47 



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 ex- 

 tinct, 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 

 nourish 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 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 comparison with mere 

 individual differences, is also applied arbitrarily, for con- 

 venience' sake. 



WIDE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES 



VARY MOST. 



Guided bj r theoretical considerations, I thought that some 

 interesting results might be obtained in regard to the nature 

 and relations of the species which vary most, by tabulating 

 all the varieties in several well-worked floras. At first this 

 seemed a simple task; but Mr. H. C. Watson, to whom I am 

 much indebted for valuable advice and assistance on this 

 subject, soon convinced me that there were many difficulties, 

 as (lid subsequently Dr. Hooper, even in stronger terms. I 

 shall reserve for a future work the, discussion of these 



