CHAP. II.] DOMINANT SPECIES VARY MOST. 39 



as steps 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 ex* 

 posed ; 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 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 

 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 inte- 

 resting 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 propor- 

 tional numbers of the varying species. Dr. Hooker permits me 

 to add that after having carefully read my manuscript, and ex- 

 amined the tables, he thinks that the following statements are 

 fairly well established. The whole subject, however, treated as it 

 necessarily here is with much brevity, is rather perplexing, and 



