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 



