what would only
be a speculation.... One can in imagination summon before us a
small part at least of the circumstances that must be
contemplated and foreknown, before it can be decided what powers
and qualities a new species must have in order to enable it to
endure for a given time, and to play its part in due relation to
all other beings destined to coexist with it, before it dies
out.... It may be seen that unless some slight additional
precaution be taken, the species about to be born would at a
certain era be reduced to too low a number. There may be a
thousand modes of ensuring its duration beyond that time; one,
for example, may be the rendering it more prolific, but this
would perhaps make it press too hard upon other species at other
times. Now if it be an insect it may be made in one of its
transformations to resemble a dead stick, or a leaf, or a
lichen, or a stone, so as to be somewhat less easily found by
its enemies; or if this would make it too strong, an occasional
variety of the species may have this advantage conferred on it;
or if this would be still too much, one sex of a certain
variety. Probably there is scarcely a dash of colour on the wing
or body of which the choice would be quite arbitrary, or which
might not affect its duration for thousands of years. I have
been told that the leaf-like expansions of the abdomen and
thighs of a certain Brazilian Mantis turn from green to yellow
as autumn advances, together with the leaves of plants among
which it seeks its prey. Now if species come in succession, such
contrivances must sometimes be made, and such relations
predetermined between species, as the Mantis, for example, and
plants not then existing, but which it was foreseen would exist
together with some particular climate at a given time. But I
cannot do justice to this train of speculation in a letter, and
will only say that it seems to me to offer a more beautiful
subject for reasoning and reflecting on, than the notion of
great batches of new species all coming in and afterwards going
out at once[84].'

We have cited this very remarkable passage, as it affords striking
evidence of how deeply Lyell had thought on this great question at a
very early period. Nevertheless it is certain that when he wrote the
second volume of the _Principles_, he had not been abl