he second place, it is as much
opposed to the theory of natural selection, as to the theory of special
creation, that in reciprocal crosses the male element of one form should
have been rendered utterly impotent on a second form, whilst at the same
time the male element of this second form is enabled freely to fertilise
the first form; for this peculiar state of the reproductive system could
not possibly be advantageous to either species.

In considering the probability of natural selection having come into action
in rendering species mutually sterile, one great difficulty will be found
to lie in the existence of many graduated steps from slightly lessened
fertility to absolute sterility. It may be admitted, on the principle above
explained, that it would profit an incipient species if it were rendered in
some slight degree sterile when crossed with its parent-form or with some
other variety; for thus fewer bastardised and deteriorated offspring would
be produced to commingle their blood with the new species in process of
formation. But he who will take the trouble to reflect on the steps by
which this first degree of sterility could be increased through natural
selection to that higher degree which is common to so many species, and
which is universal with species which have been differentiated to a generic
or family rank, will find the subject extraordinarily complex. After mature
reflection it seems to me that this could not have been effected through
natural selection; for it could have been of no direct advantage to an
individual animal to breed badly with another individual of a different
variety, and thus leave few offspring; consequently such individuals could
not have been preserved or selected. Or take the case of two species which
in their present state, when crossed, produce few and sterile offspring;
now, what is there which could favour the survival of those individuals
which happened to be endowed in a slightly higher degree with mutual
infertility and which thus approached by one small step towards absolute
sterility? yet an advance of this kind, if the theory of natural selection
be brought to bear, must have incessantly occurred with many species, for a
multitude are mutually quite barren. With sterile neuter insects we have
reason to {187} believe that modifications in their structure have been
slowly accumulated by natural selection, from an advantage having been thus
indirectly given to the community t