42 DOMIXANT SPECIES VARY MOST. [Chap. II. 



towards more strongly-marked and permanent varieties ; and at the 

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

 from one stage of difference to another may, in many 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 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 througnout 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 them 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 

 distinct and more fluctuating forms. The term variety, again, in 

 comparison with mere individual differences, is also applied arbi- 

 trarily, for convenience' sake. 



Wide-ranging, much-diffused, and common Species vary most. 



Guided by theoretical considerations, I thought that some in- 

 teresting 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 did subsequently Dr. 

 Hooker, 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. Hooker permits me to add, 

 that after having carefully read my manuscript, and examined the 





