Page:Philosophical Review Volume 14.djvu/373

357 only is to be seen, and it is wholly unconnected with the original type. Fluctuations are subject to reversions, mutations are not.

VI. The mutations take place in nearly all directions. 'Some may be favorable, others detrimental, many of them without significance either way.' 'How these differences originate has nothing to do with the theory of natural selection nor with the struggle for life.' But if the useful directions are given a number of times, a cumulation and a progression are possible, which give the effect of an apparent predisposition.

These facts necessitate a distinction in 'natural selection.' Lectures XXV-XXVIII. There is selection of two kinds, between species and between varieties. In the first there is a reduction in the number of species. Some thrive and multiply, others not. New mutations appear, and the sifting-selection occurs. This explains the manifold highly complicated structure which strikes the beginner as adaptation. The second, intra-species selection, crowns the first. It brings isolated forms to the highest possible degree of usefulness, adapts strains to local conditions, produces local races. It produces lateral branches, but no main stems of pedigree, no lasting improvement.

One question of great importance remains to be discussed: Is mutability a temporary and periodic or a permanent condition? If the latter, then it has no beginning and is not due to external circumstances. If the former, there was a beginning due to an external cause; although the amount and direction may be 'assumed to be due to internal causes.' Which of these agrees best with the facts of species not at present mutable? By the second view this would mean for these the 'loss of the capacity for further development.' By the first, this capacity could be regained and all lines of the genealogical tree might progress. Experience must decide between the two main theories. Now, however, it shows a previous state of mutability in a large number of polymorphous genera; while, on the other hand, many facts plead in favor of the constancy of species. The mutation theory conciliates the two. Reducing the change-ability of the species to distinct and probably short periods, it allows for the constancy so emphasized by the systematist, and for the descent through modification of the evolutionist. It grants the present constancy of the vast majority of living forms, and only claims the exceptional occurrence of definite changes, while the indications are that these periods of stability and mutability alternate more or less regularly with one another.

At certain periods, then, new species arise in considerable numbers,