S8 ORIGIN OF SPECIES 



impressions. As he extends the range of his observations, he 

 will meet with more cases of difliculty ; for he will encounter 

 a greater number of closely-allied forms. But if his observa- 

 tions be widely extended, he will in the end generally be 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 naturalists. When 

 he comes to study allied forms brought from countries not 

 now continuous, in which case he cannot hope to find inter- 

 mediate links, he will be compelled to trust almost entirely to 

 analogy, and his difificulties 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 series ; 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 towards such slight varieties as 

 are barely thought worth recording in works on natural his- 

 tory. And I look at varieties which are in any degree more 

 distinct and permanent, as steps towards more strongly- 

 marked and permanent varieties; and at the latter, as lead- 

 ing 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 an- 

 other, 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 vari- 

 ety 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 



