42 THE ORIGIN OF SPECIES 



latter, as leading to sub-species and then to species. The passage 

 from one stage of difference to another may, in some 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 ex- 

 plained, 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 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 convenience, 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 dis- 

 tinct and more fluctuating forms. The term variety, again, in com- 

 parison with mere individual differences, is also applied arbitrarily, 

 for convenience' sake. 



WIDE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES 

 VARY MOST 



Guided by theoretical considerations, I thought that some inter- 

 esting results might be obtained in regard to the nature and rela- 

 tions 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 did subsequently Dr. Hooper, even in 

 stronger terms. I shall reserve for a future work the discussion of 

 these difficulties, and the tables of the proportional numbers of the 

 varying species. Dr. Hooper permits me to add that after having 

 carefully read my manuscript, and examined the tables, he thinks 



