ORIGIN OF ADAPTATIONS AND OF SPECIES 245 



effects of use and disuse are important in organic evolution. 

 (3) Effects of climatal influences and of nutrition are fre- 

 quently adaptive and often transmissible, as experiments have 

 proved. There is, however, much difference of opinion as to 

 the precise way in which these effects are transmitted. 



Incidental Adaptations. Many leaf-eating caterpillars 

 and grasshoppers are green from the presence of chlorophyll 

 in their bodies; they owe their color directly to their food. 

 Now it may be admitted that this green color is often protec- 

 tive, without admitting that the color was acquired for that 

 purpose. In the case of green leaf-mining caterpillars, cer- 

 tainly, the color appears to be superfluous for protective pur- 

 poses. Even variegated protective coloration may be simply 

 a direct effect of the surrounding kinds of light, as Poulton 

 proved. 



Again, take the various tropisms, described in another 

 chapter. Often they are adaptive and often they are not ; but 

 they occur inevitably, whether they result advantageously or 

 not. It is too much to say that a useful structure or function 

 appeared because of its usefulness. It first appeared, and then 

 proved to be either useful or not useful. If useful, a structure 

 may save the life of its possessor and possibly be transmitted to 

 the next generation ; if harmful, it is self-eliminating. 



2. SPECIES 



Modifications arise, and are either useful or not to their 

 possessors. For the systematist who aims merely to distin- 

 guish one species from another, this distinction matters but 

 little. To the biologist, however, the difference is an essential 

 one, and he draws a line between specific peculiarities that are 

 adaptive and those that are not adaptive. The origin of 

 species and the origin of adaptations are by no means the 

 same thing. 



Darwin's Origin of Species. At the time Darwin's great 

 work was written, its immediate purpose was to demonstrate 

 a process of organic evolution; and this object was accom- 



