Page - 68 - in The Origin of Species

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