thesis yet suggested for explaining the
origin of species.

From what has been said of the changes which are always going on in the
habitable surface of the world, and the manner in which some species are
constantly extending their range at the expense of others, it is evident
that the species existing at any particular period may, in the course of
ages, become extinct one after the other.

If such, then, be the law of the organic world, if every species is
continually losing some of its varieties, and every genus some of its
species, it follows that the transitional links which once, according to
the doctrine of transmutation, must have existed, will, in the great
majority of cases, be missing. We learn from geological investigations
that throughout an indefinite lapse of ages the whole animate creation
has been decimated again and again. Sometimes a single representative
alone remains of a type once dominant, or of which the fossil species
may be reckoned by hundreds. We rarely find that whole orders have
disappeared, yet this is notably the case in the class of reptiles,
which has lost some orders characterised by a higher organisation than
any now surviving in that class. Certain genera of plants and animals
which seem to have been wholly wanting, and others which were feebly
represented in the Tertiary period, are now rich in species, and appear
to be in such perfect harmony with the present conditions of existence
that they present us with countless varieties, confounding the zoologist
or botanist who undertakes to describe or classify them.

We have only to reflect on the causes of extinction, and we at once
foresee the time when even in these genera so many gaps will occur, so
many transitional forms will be lost, that there will no longer be any
difficulty in assigning definite limits to each surviving species. The
blending, therefore, of one generic or specific form into another must
be an exception to the general rule, whether in our own time or in any
period of the past, because the forms surviving at any given moment will
have been exposed for a long succession of antecedent periods to those
powerful causes of extinction which are slowly but incessantly at work
in the organic and inorganic worlds.

They who imagine that, if the theory of transmutation be true, we ought
to discover in a fossil state all the intermediate links by which the
most dissimilar types have been formerly connected together, expect