lesser differences become augmented into the greater difference? How do
varieties, or as I have called them incipient species, become converted
into true and well-defined species? How has each new species been adapted
to the surrounding physical conditions, and to the other forms of life on
which it in any way depends? We see on every side of us innumerable
adaptations and contrivances, which have justly excited the highest
admiration of every observer. There is, for instance, a fly (Cecidomyia
(Introduction/3. Leon Dufour in 'Annales des Science. Nat.' (3rd series,
Zoolog.) tome 5 page 6.)) 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,--has been briefly treated in my 'Origin of Species.' It
was there shown 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