DOUBTFUL SPECIES. 49 



able to make up his own mind; but he will succeed in this 

 at the expense of admitting much variation, and the truth 

 of this admission will often be disputed by other natural- 

 ists. When he comes to study allied forms, brought from 

 countries not now continuous, in which case he cannot 

 hope to find intermediate links, he will be compelled to 

 trust almost entirely to analogy, and his difficulties will 

 rise to a climax. 



Certainly no clear line of demarcation has as yet been 

 drawn between species and sub-species— that is, the forms 

 which in the opinion of some naturalists come very near 

 to, but do not quite arrive at, the rank of species; or, 

 again, between sub-species and well-marked varieties, or 

 between lesser varieties and individual differences. These 

 differences blend into each other by an insensible sejies; 

 and a series 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 1 look at varieties wliich are in any degree 

 more distinct and permanent, as steps toward more 

 strongly marked and permanent varieties; and at the lat- 

 ter, 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 im- 

 portant 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 behef is justifiable must 

 be judged by the weight of the various facts and considera- 

 tions 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. WoUaston with 

 the varieties of certain fossil land-shells in Madeira, and 



