_facts_, and also the fairly certain _inferences_ of Evolution, are then
accounted for.


[Footnote 1: Pigeon fanciers know that when they have once obtained, by
crossbreeding and selection, a particular form or feather, the utmost
care is needed to preserve it. If the parents are not selected the
progeny wilt gradually revert towards the original wild pigeon type.]

We have neither by revelation nor physical discovery an exact _scheme_
of all the types, nor which of the elementary forms were destined to
remain unchanged throughout. But some scheme of created types we surely
have. Whether what we call _species_[1] are all types or not, we cannot
say; probably not. All we can be sure of is that there are definite
lines somewhere. We see the sterility of some hybrids, for instance,
which would seem to indicate that while some forms can conjugate and
their offspring remain fertile, others (approaching, as it were, the
verge of separation) give rise to hybrids which are or not absolutely
sterile,[2] according as they approach, or are more remote from, the
designed barrier-line. And at that point the separation is insuperable.
Certain forms of _Carnivora_ and _Ungulata_ seem to be for ever
apart--not only the two great orders, but even subdivisions within them.
Reptiles and birds, on the other hand, unlike as they at first sight
seem, have no type line drawn to separate them; that, at least, is one
of the more recent conclusions of biological science.


[Footnote 1: It should be borne in mind that what we call a _species_ as
distinct from a mere variety, is a more or less arbitrary or provisional
thing dependent on the state of science for the time. Species are