y lessened fertility to absolute sterility. It may be admitted
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 high degree which is common with 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. Take the
case of any two species which, when crossed, produced 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 believe that modifications in
their structure and fertility have been slowly accumulated by natural
selection, from an advantage having been thus indirectly given to the
community to which they belonged over other communities of the same
species; but an individual animal not belonging to a social community,
if rendered slightly sterile when crossed with some other variety, would
not thus itself gain any advantage or indirectly give any advantage
to the other individuals of the same variety, thus leading to their
preservation.

But it would be superfluous to discuss this question in detail: for with
plants we have conclusive evidence that the sterility of crossed species
must be due to some principle, quite independent of natural selection.
Both Gartner and Kolreuter have proved that in genera including numerous
species, a series can be formed from species which when crossed yield
fewer and fewer seeds, to species which never produce a single seed, but
yet are affected by the pollen of certain other species, for the germen
swells. It is here manifestly impossible to select the more sterile
individu