490 THE POPULAR SCIENCE MONTHLY. 



Darwinian factor of modification, and a wliole volume of illustra- 

 tions may be drawn from entomology; for in no class is the con- 

 test more severe, whether with plants, or with other animals, or 

 with one another, than in insects. In no other field of biology, 

 for instance, have the physical conditions resulted in such infinite 

 diversity of form and habit fitted, whether for earth, air, or water, 

 and often for all in the same individual; so, also, in no other 

 field is parasitism carried to such a degree, or are the purely 

 adaptive structures due to this interaction so varied or so remark- 

 able. The entomologist who goes beyond the " dry bones " of his 

 science is inevitably a Darwinian. 



In this category must also be included that interrelation be- 

 tween insects and plants which has eventuated in the so-called 

 carnivorous plants, and that still more wonderful interaction 

 between flowers and insects by which each has modified the other, 

 and the facts of which have been so. untiringly observed and so 

 well set forth by a number of writers from Sprengel's day to this, 

 and by none more successfully than by Darwin himself. These 

 are plainly inexplicable on external conditions acting on masses 

 alike, and are meaningless enigmas except on the theory of natural 

 selection, or some supra-natural and dogmatic gospel. 



We are thus led, through this last, from the external to the 

 internal factors in evolution, or those of a physiological and 

 psychical nature. In these, natural selection is the key which, 

 so far, best unlocks their meaning, and shows how they have acted 

 in the formation of species and the less fundamental of the great 

 groups. In considering them it is hardly necessary to discuss 

 their relative importance as compared with the external con- 

 ditions, though it may be remarked that they are the factors 

 which have induced the great variety of adaptive forms and 

 minor differentiations, while the external conditions have gov- 

 erned the formation of the great and more comprehensive types 

 of structure. 



Darwin was led to give more importance toward the end than 

 he had originally done to some of these internal factors, and es- 

 pecially to functionally produced modifications. In the " Descent 

 of Man " he says that he did not sufiiciently consider variations 

 " which so far as we can at present judge are neither of benefit nor 

 injurious ; and this I believe to be one of the greatest oversights 

 I have yet detected in my work." And in the sixth edition of 

 the " Origin " he frankly admits that he had omitted in other edi- 

 iions to consider properly the frequency and importance of modi- 

 fications due to spontaneous variability. He further refers to mor- 

 phologic differences, which may have become constant through 

 the nature of the organism and the surrounding conditions rather 

 .than through natural selection, since they do not affect the wel- 



