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
constantly being lumped together by some and separated by others. It
follows most probably, that while some species are really types--i.e.,
one can never pass into the other and lose its essentials, unless it is
destined to disappear (like the pterodactyle), not being wanted in the
whole scheme--other species are really only varieties, and maybe lost or
modified without limit.]
[Footnote 2: We may well regard the mule as a peculiar form just such as
the evolutionist would rejoice to see: here is a modified species, which
has qualities different from those of either of the parent stock, and
well fitted "to struggle for existence." Yet this modified race would,
if left to itself, die out.]
In other cases where variation has occurred, and especially when it is
artificially--i.e., by the aid of selective breeding--caused or
favoured, there is the constant tendency to _revert_, which is at once
intelligible if there is a type scheme to be maintained.
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