The view that species are wholly arbitrary groups is not accepted by all naturalists. De Vries especially lays stress on the point that species are objective. Every species, he be lieves, is made up of a number of specific char acters, each of which depends upon a certain chemical particle or aggregation of molecules, which perpetuates itself and multiplies by self division. So long as the descendant chemical particles remain like the parental ones, so long the species persists. These particles may, how ever, suddenly change through some cause, not precisely known, but probably environmental; and with a change in one of these particles goes a change in the adult specific quality that it con trols, and also of the 'correlated' qualities. The consequence is that a form with many new quali ties suddenly arises, and this form we may call a new species. De Vries has studied (Die Mato tionstheorie, 11001) variable species in the field —for example, the evening primrose—and finds that new forms, clearly marked off from the original. typical ones, are constantly arising. They are, from the beginning, distinct and fully formed species. There are, however, other theo ries to account for that 'distinctness' or isola tion which is not infrequently found. One is that species originated in isolated situations.
A group of land animals, getting by acci dent upon an island and varying normally, will in time come to be quite dissimilar from the forms on the mainland, because, being isolated, any pe culiarities that may crop out among them will not be swamped by intercrossing with the main mass of the species. Thus, we find in the Gala pagos islands that each island has a peculiar species of lizard of a particular genus, and only one species occurs on each island. The same is true of certain sparrows on these islands. Even on islands nearer the mainland, like Nantucket or Fire island, on the south side of Long Island, there are peculiar forms—incipient species. it is the isolation which has permitted them to arise; even a less degree of isolation may per mit a difference to grow up. Thus, Darwin tells this story: "The two flocks of Leicester sheep kept by Mr. Buckley and Mr. Burgess have been purely bred from the original stock of Mr. Bake well for upward of fifty years. There is not a suspicion existing in the mind of any one at all acquainted with the subject that the owner of either of theta has deviated in any one instance from the pure blood of 1\1r. Bakewell's flock; and yet the difference between the :heti% possessed by these two gentlemen is so great that they have the appearance of being quite different va rieties." Darwin's explanation of the lack of intergrades between many species differs from the foregoing. •and depends on the assumption that, of the descendants of any species. those
that are most unlike are most apt to survive; so that where the number of forms (varieties) is very great, the intermediate ones (the inter grades) are pushed to the wail and are extin guished. lu Darwin's words: "As in each fully stocked country natural select ion necessarily acts by the selected form having some advantage in the struggle for life over other forms, there will be a constant tendency in the improved descendants of any one species to supplant and exterminate, in each stage of descent, their prede cessors and their original progenitor. For it should be remembered that the competition will generally be most severe between those forms which are most nearly related to each other in habits, constitution, and structure. Hence all the intermediate forms between the earlier and later states—that is, between the less and more improved states of the same species—as well as the original parent species itself, will generally tend to become extinct. So it probably will be with many whole collateral lines of descent, which will be conquered by later and improved lines." This striking passage of Darwin deserves an experimental or statistical test. Until that is had, it can only stand as an excellent illustra tion of Darwin's logical treatment of special difficulties in his theory.
Another important attribute of a species is tbat its members are frequently sterile, with even closely related species. It would, however, be a mistake to assume that, on the one hand, all species, or even the hybrid offspring of crossed species, are always sterile. It is likewise an error to suppose that, on the other hand, sterility is confined to crosses between species. Breeders of animals are accustomed to find a considerable percentage of sterility between members of the same species; and when the members are close blood-relations, the percentage becomes very high. Nevertheless, the sterility of hybrids (crosses between species) is so common that Cuvier gave it as the sure criterion of species. For further facts and explanations of this question, see I I ICBM MTV.
Finally, species are adapted to the conditions in which they live. By many this characteristic is considered a fundamental one; so that, when we have explained how their adaptations arise, we have explained how the species arose. It may he worth while, here, to state that there is a lack of unanimity in respect to the adaptive nature of all specific characters. Those who as sert such adaptation insist that it cannot be denied that any given apparently useless organ may not be useful, or may not have been useful in some period of the animal's life, or that of its ancestors. See Evoix-rioN; NATI:RAI. SELEC TION and similar titles, and the books men tioned thereunder.