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THE FORMS OF PROSE LITERATURE
CHARLES DARWIN: THE FORMATION OF SPECIES
From "The Origin of Species," London, 1875, ch. iv. pp. 90-97.
This first selection shows how much assistance a diagram may be in putting before your mind a swarm of facts which you must consider all at the same time. They must be simplified so that you shall look at only a single aspect of each: here Darwin discusses only the comparative divergence of species, leaving out of view all their other manifold aspects. I have discussed this subject fully in the Introduction, p. 29. It is worth pointing out, however, the way in which Darwin sums up his results in the last paragraph of the selection; and he not only sums them up, but discusses the facts in the new light which he has gained by this consideration of a special relation.
AFTER the foregoing discussion, which has been much compressed, we may assume that the modified descendants of any one species will succeed so much the better as they become more diversified in structure, and are thus enabled to encroach on places occupied by other beings. Now let us see how this principle of benefit being derived from divergence of character, combined with the principles of natural selection and of extinction, tends to act.
The accompanying diagram will aid us in understanding this rather perplexing subject. Let A to L represent the species of a genus large in its own country; these species are supposed to resemble each other in unequal degrees, as is so generally the case in nature, and as is represented in the diagram by the letters standing at unequal distances. I have said a large genus, because as we saw in the second chapter, on an average more species vary in large genera than in small genera; and the varying species of the large genera present a greater number of varieties. We have, also, seen that the species, which are the commonest and the most widely diffused, vary more than do the rare and restricted species. Let (A) be a common, widely-diffused, and varying species, belonging to a genus large in its own country. The branching and diverging dotted lines of unequal lengths proceeding from (A), may represent its varying offspring. The variations are supposed to be extremely slight, but of the most diversified nature; they are not supposed all to appear simultaneously, but often after long intervals of time; nor are they all supposed to endure for equal periods. Only those variations which are in some way profitable will be preserved or naturally selected. And here the importance of the principle of benefit derived from divergence of character comes in; for this will generally lead to the most different or divergent variations (represented by the outer dotted lines) being preserved and accumulated by natural selection. When a dotted line reaches one of the horizontal lines, and is there marked by a small num