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- 
