nd of
every observer the highest admiration. There is, for instance, a fly
(Cecidomyia)[3] which deposits its eggs within the stamens of a
Scrophularia, and secretes a poison which produces a gall, on which the
larva feeds; but there is another insect (Misocampus) which deposits its
eggs within the body of the larva within the gall, and is thus nourished by
its living prey; so that here a hymenopterous insect depends on a dipterous
insect, and this depends on its power of producing a monstrous growth in a
particular organ of a particular plant. So it is, in a more or less plainly
marked manner, in thousands and tens of thousands of cases, with the lowest
as well as with the highest productions of nature.

This problem of the conversion of varieties into species,--that is, the
augmentation of the slight differences characteristic of varieties into the
greater differences characteristic of species and genera, including the
admirable adaptations of each being to its complex organic and inorganic
conditions of life,--will form the main subject of my second work. We shall
therein see that all organic beings, without exception, tend to increase at
so high a ratio, that no district, no station, not even the whole surface
of the land or the whole ocean, would hold the progeny of a single pair
after a certain number of generations. The inevitable result is an
ever-recurrent Struggle for Existence. It has truly been said that all
nature is at war; the strongest ultimately prevail, the weakest fail; and
we well know that myriads of forms have disappeared from the face of the
earth. If then organic beings in a state of nature vary even in a slight
degree, owing to changes in the surrounding {6} conditions, of which we
have abundant geological evidence, or from any other cause; if, in the long
course of ages, inheritable variations ever arise in any way advantageous
to any being under its excessively complex and changing relations of life;
and it would be a strange fact if beneficial variations did never arise,
seeing how many have arisen which man has taken advantage of for his own
profit or pleasure; if then these contingencies ever occur, and I do not
see how the probability of their occurrence can be doubted, then the severe
and often-recurrent struggle for existence will determine that those
variations, however slight, which are favourable shall be preserved or
selected, and those which are unfavourable shall be destroyed.

Thi