220 
[October 
The only valid practical criterion of specific distinctness is the gene¬ 
ral non-existence, either actually ascertained or analogically inferred, 
of intermediate grades in the distinctive characters, whence we may 
reasonably conclude that the two supposed species are distinct, i. e. 
that they do not now in general mix sexually together, or if geographi¬ 
cally separated that they would not do so supposing them to be placed 
in juxtaposition. Whether they are descended from common parents 
which ages and ages ago mixed sexually together, is another and a 
very different question, which concerns the speculative philosopher not 
the practical naturalist. They may even now mix sexually together 
in some few rare instances, as I have recorded to be the case with 
Coccinella ahdominalis Say, and Ghilocorus hivulnerus Muls. ^ (Proc. 
Ent. Soc. Phil. I, p. 351,) and yet if they do not commonly and habit¬ 
ually mix together the species will remain distinct. Hence all experi¬ 
ments on artificial hybridization seem to me to prove nothing as to the 
distinctness of species, unless they are conducted, as they necessarily 
cannot be, on the same gigantic scale as that upon which Nature 
works. We may and do mix the horse and the ass and produce a mule 
or a hinny, but what naturalist believes that if a herd of wild asses 
were transported to the plains of Mexico they would mix habitually 
with the mustangs and produce all the intermediate grades ? Every 
field-entomologist is aware of the fact that 5 or 6 very closely allied 
species, e. g. of Cicindela, will often be found on a piece of ground 100 
yards square, and yet that every one of them keeps perfectly distinct. 
A similar case occurs in the genus Gromphus, of which 10 perfectly 
distinct but closely-allied species co-exist in the neighborhood of Rock 
Island, Ill., three of which, as already recorded, (Ill. Pseudon. p. 396,J 
I have seen coming out of the water together in the pupa state on one 
and the same day, and on one and the same spot of ground. Imme¬ 
diately that we assume any other criterion of specific distinctness than 
the general non-existence in a state of nature of the intermediate grades, 
either proved by actually examining numerous specimens or inferred 
from the analogy of allied species, all is chaos and confusion. The 
formerly received dogma that hybrids are incapable of further propa¬ 
gation, has been proved by Rev. Herbert and others to be utterly 
false in the Vegetable Kingdom, and is probably false in the Animal 
Kingdom. Hence may be seen the importance of describing species. 
