46 
VARIATIO^iT UNDER NATURE. 
Chap. II. 
by slow degrees: yet quite recently Mr. Lubbock has 
shown a degree of variability in these main nerves in 
Coccus, which may almost be compared to the irregular 
branching of the stem of a tree. This philosophical 
naturalist, I may add, has also quite recently shown 
that the muscles in the larvae of certain insects are 
very far from uniform. Authors sometimes argue in 
a circle when they state that important organs never 
vary; for these same authors practically rank that cha¬ 
racter as important (as some few naturalists have honestly 
confessed) which does not vary; and, under this point 
of view, no instance of an important part varying will 
ever be found: but under any other point of view many 
instances assuredly can be given. 
There is one point connected with individual differ¬ 
ences, which seems to me extremely perplexing: I 
refer to those genera which have sometimes been called 
protean” or ^^polymorphic,” in which the species present 
an inordinate amount of variation; and hardly two natu¬ 
ralists can agree which forms to rank as species and 
which as varieties. We may instance Eubus, Eosa, and 
Hieracium amongst plants, several genera of insects, and 
several genera of Brachiopod shells. In most polymorphic 
genera some of the species have fixed and definite cha¬ 
racters. Genera which are polymorphic in one country 
seem to be, with some few exceptions, polymorphic in 
other countries, and likewise, judging from Brachiopod 
shells, at former periods of time. These facts seem to 
be very perplexing, for they seem to show that this land 
of variability is independent of the conditions of life. 
I am inclined to suspect that we see in these poly¬ 
morphic genera variations in points of structure which are 
of no service or disservice to the species, and which con¬ 
sequently have not been seized on and rendered definite 
by natural selection, as hereafter will be explained. 
