down the back, while
the tigers of Java and Sumatra differ in many important respects from
the tigers of Northern Asia. So lions vary; so birds vary; and so, if
you go further back and lower down in creation, you find that fishes
vary. In different streams, in the same country even, you will find the
trout to be quite different to each other and easily recognisable by
those who fish in the particular streams. There is the same differences
in leeches; leech collectors can easily point out to you the differences
and the peculiarities which you yourself would probably pass by; so with
fresh-water mussels; so, in fact, with every animal you can mention.

In plants there is the same kind of variation. Take such a case even as
the common bramble. The botanists are all at war about it; some of
them wanting to make out that there are many species of it, and others
maintaining that they are but many varieties of one species; and they
cannot settle to this day which is a species and which is a variety!

So that there can be no doubt whatsoever that any plant and any
animal may vary in nature; that varieties may arise in the way I have
described,--as spontaneous varieties,--and that those varieties may be
perpetuated in the same way that I have shown you spontaneous varieties
are perpetuated; I say, therefore, that there can be no doubt as to the
origin and perpetuation of varieties in nature.

But the question now is:--Does selection take place in nature? is there
anything like the operation of man in exercising selective breeding,
taking place in nature? You will observe that, at present, I say nothing
about species; I wish to confine myself to the consideration of the
production of those natural races which everybody admits to exist. The
question is, whether in nature there are causes competent to produce
races, just in the same way as man is able to produce by selection, such
races of animals as we have already noticed.

When a variety has arisen, the CONDITIONS OF EXISTENCE are such as to
exercise an influence which is exactly comparable to that of artificial
selection. By Conditions of Existence I mean two things,--there are
conditions which are furnished by the physical, the inorganic world,
and there are conditions of existence which are furnished by the organic
world. There is, in the first place, CLIMATE; under that head I include
only temperature and the varied amount of moisture of particular places.
In the next pla