Page:Popular Science Monthly Volume 62.djvu/491

Rh for as easily by accepting exceedingly slow changes as the causes of the formation of species, as they would be were we to consider them due to shocks occurring but once in a protracted period. Darwin fully realized this and considered the doubt upon this subject as one of the weakest points of his theory.

Darwin, as did many since, compared the origin of species in nature to the methods, ordinarily used in agriculture, to obtain improved races of plants and animals. On this subject much confusion exists. Horses, for instance, are improved principally by crossing with specimens of a superior race, which specimens more or less fully transfer the good qualities of the race to the descendants. But it is certain that in nature the species did not originate in this manner, at least not as a general rule. Improved races are obtained only by careful and constant selection in one direction. This bears a great resemblance to the origin of species, but there is this objection, that such a race would never be independent of selection; as soon as selection ceases, the good qualities disappear. Species and subspecies, even true varieties, on the other hand, are totally independent of the mother species; neither in nature nor in cultivation do they return to the old type, either by a change in environmental conditions or by the cessation of a selection; always provided of course, that accidental crossing is impossible.

The experience yielded by agriculture would lead one to consider a gradual transition from one species into another improbable. They point to a distinct difference between gradually improved races and those suddenly formed, so-called varieties. The former bear no resemblance, the others a resemblance in all respects to wild species.

During the last few decades several writers have expressed themselves more or less strongly against the conception of a gradual origin of species. In America Cope was the one to set the example. Among paleontologists Dollo, among zoologists Bateson, and recently among botanists Korschinsky declared themselves in favor of the doctrine of the discontinuity of the natural ancestral trees. But their opinions have not been sharply defined and formulated and are based upon an acquaintance with facts not much larger than that commanded by Darwin himself. Hence their small influence and the small progress made by their convictions. Hence the American paleontologist Scott, a devoted adherent of Cope's doctrine, deemed it necessary to defend this doctrine against Bateson 's book. For his conception of discontinuity is entirely different from that of Bateson. They are both dissatisfied with the reigning views on the origin of species by gradual variability, but in its place each wishes to put an entirely different conception.