238 EN TOMOLOGY 
fact that some variations which appear in successive genera- 
tions are due not to inheritance but to the direct action of the 
environment on each successive generation; also to the fact 
that some structural changes may have been brought about by 
selection of some sort, rather than by inheritance. Are the 
results of use or disuse or mutilation inheritable? It has not 
been proved as yet that these “acquired characters” are 
transmissible. On the other hand, experiments show that 
some organisms can become acclimatized to unusual degrees 
of heat, density, etc., through inheritance, in cases where selec- 
tion does not enter into the problem. Much of the confusion 
attending the discussion of “the inheritance of acquired char- 
acters’? has been due to disagreements as to what is meant by 
the term “acquired characters.” (3) What are the secondary 
influences that have brought about the evolution of structures ? 
Of these influences, natural selection and isolation are by far 
the most important; while in some instances extensive struc- 
tural adaptations have arisen spontaneously, without a long 
course of evolution. 
Natural Selection.—The more intricate adaptations of 
organism to environment, however, are for the most part inex- 
plicable without the aid of Darwin’s and Wallace’s theory of 
natural selection. After almost fifty years of searching criti- 
cism and even violent opposition, this theory, though modified 
in some respects, remains essentially as it was formulated, and 
is at present the working hypothesis of most naturalists. This 
doctrine is here outlined in its several factors. 
Excessive Multiplication.—Any one species of animal or 
plant, were its multiplication unchecked, would soon cover the 
earth. The progeny of a single aphid in ten generations, as 
calculated by Huxley, would * contain more ponderable sub- 
stance than five hundred millions of stout men; that is, more 
than the whole population of China.”’ The hop aphid (Phor- 
odon humuli), studied by Riley, has thirteen generations a 
year, consisting entirely of females up to the last generation. 
Assuming that each female produces 100 young and that the 
