ON THE CAUSES OF VARIATION. 489
ature in directly inducing specific changes. Weismann, in his remarkable " Studien der Descendenz - Theorie/' concludes that differences of specific value can originate only through the direct action of external conditions, and that allied species and genera, and even entire families, are modified in the same direction by similar external inducing causes. In Semper's "Animal Life" (1877) we have the best systematized effort to bring together the direct causes of variation, and no one who has read through its pages can doubt the direct modifying influences of nutrition, light, temperature, water at rest and in motion, atmosphere still or in motion, etc., or question his conclusion that no power which is able to act only as a selective and not as a transforming influ- ence can ever be exclusively put forth as a causa efficiens of the phenomena. Kolliker, in 1873, wrote : " Manifold external con- ditions, when they operate on eggs undergoing their normal de- velopment, on larvse or other early stages of animals, and on the adult forms, have produced in them partly progressive and partly regressive transformations " ; and recognized as most important forces, nutrition, light, and heat. Indeed, the direct action of environment must have been, as Spencer puts it, " the primordial factor of organic evolution."
In so far as it offers evidence, entomology confirms the conclu- sions of the writers in other dei)artments of natural history, above referred to, and offers a host of most conclusive proofs of the direct action of the physical and chemical factors which I have enumerated. Justice, however, could not be done to the facts within the limits of an address of this kind, and I pass on to some of the other factors.
It is among what I have called the vital or organic conditions of variation that natural selection has fullest sway, and, as they have been so ably expounded by Darwin and others, they may be dealt with in few words.
Interaction of Organisms. — The productions, as a whole, of greater areas will, whenever they get an opportunity, conquer those of lesser areas, and in this broad sense the interaction of organisms may be said to have had no special modifying power, however great its influence may have been, and is yet, in inducing the survival of the fittest, or in bringing about the present geo- graphical distribution of species. The consequences of enforced migration and of isolation are best considered when dealing with the physical conditions, because they must influence modification of masses rather than of individuals, and either substitute one type for another or remove competing or differentiating influ- ences. But, in the more restricted sense, i. e., the interaction of organisms occupying the same ground — the struggle for existence, in other words, between direct competing organisms— is a prime