194 The Question of Organic Evolution. [April, 
multiplied undisturbed, there is no doubt that in the course 
of ages it would have reached a point at which it would no 
longer have been capable of productive intercourse with the 
original stock, as we have seen is the case with the tame 
guinea-pig. Let us then suppose that its habitat had been 
visited by Cuvier and a band of his disciples. Unless spe- 
cially informed of its origin, would not these sages calmly 
note it down as a ‘‘ good species ?” 
The origin of parasites, internal and external, has long 
been a stumbling block to naturalists. Were these beings 
created simultaneously with the species they infest, or have 
they been produced by some subsequent miracle? Such 
were the questions asked by the old school. It was admitted 
that these vermin are not now found living otherwise than 
parasitically ; that their present structure unfits them for an 
independent life, and that many of them are peculiar to some 
one particular animal, or even to some part of an animal. 
They were the betes noires of teleologists, none of whom, we 
believe, has yet made the tape-worm or the Trichina the 
subject of an extra Bridgewater Treatise. But in the light 
of the doctrine of evolution the difficulty vanishes. In 
successive generations these animals have become gradually 
modified to the places they inhabit and the parts they play ; 
so that to trace what were their original forms and attributes 
may now be beyond the reach of the human intellect. Do 
not similar considerations—the gradual modification of 
certain low organisms, animal or vegetable, or, perhaps, 
doubtful—explain the origin of such diseases as rabies, 
syphilis, and variola, which in our days never occur except 
in consequence of transmission from one animal to another, 
and which we cannot conceive as primordial ? 
We are thus naturally led to one of the cardinal doctrines 
of the old Natural History. Whoof us entomologists, when 
curiously examining some specimen, has not been accosted, 
perhaps by a child, perhaps by a grown-up outsider, with 
the question: *‘ What is the use of that creature, and 
to what end was it made?” On our replying, as might 
often be the case, that to the best of our belief and know- 
ledge it was of no use at all, but decidedly pernicious, have 
we not been told that “every living thing was made for 
some good end?” ‘This doctrine is somewhat inconsistent 
in the mouths of sparrow- and rook-shooting farmers, aphis 
and red-spider-poisoning gardeners, and a very large majority 
of the human race. It is, too, decidedly obscure. Are all 
animals useful to man, or to the universe at large? If we 
take the former view, we shall generally have to strike 
