G2 DOMINANT SPECIES VARY MOST. Chap. II. 



individual differences. These differences blend into each other 

 in an insensible scries ; and a seiies impresses the mind with 

 the idea of an actual passage. 



Hence, I look at individual differences, though of small 

 interest to the systematist, as of the highest importance for 

 us, as being the first steps toward such slight varieties as are 

 barely thought worth recording in works on natural history. 

 And I look at varieties which are in any degree more distinct 

 and permanent, as steps toward more strongly-marked and 

 permanent varieties ; and at the latter, as leading to sub- 

 species or species. The passages from one stage of difference 

 to another may, in some cases, be the simple result of the 

 long-continued action of different physical conditions ; but in 

 most cases they may be attributed to the gradual accumulative 

 'action of natural selection, as hereafter to be more fullj" ex- 

 plained, on fluctuating variability. • Hence a well-marked 

 variety may be called an incipient species ; but whether this 

 belief is justifiable must be judged of by the general weight 

 of the facts and considerations given throughout this Avork. 



It need not be supposed that all varieties or incipient spe- 

 cies necessarily 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 immbers the parent-species, it would then rank as 

 the species, and the species as the variety ; or it might come 

 1o supplant and exterminate the parent-species ; or both might 

 coexist, 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 tlie 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 arbitrarily, and for mere convenience' sake. 



Wide-ranging^ much-d {fused, and Commo7i Siyecics, vary 



most. 



Guided by theoretical considerations, I thought that some 

 interesting results might be obtained in regard to the nature 



