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.

If there were a series of created types, there may naturally have been
what I may call sub-types; which would be certain well-marked stages on
the way to the final form. Such sub-type forms would naturally occur at
different ages, and being marked would show their place in the scale,
and their connection with the ultimate perfect form. Such a possibility
would exactly account for the series of _Eohippus, Hipparion_, and
horse, which we have already instanced; and still more so for the rise
and disappearance of the great Mesozoic Saurians when their object was
fulfilled. Deny guidance and type, and everything becomes confused. Why
should variation take certain directions? how comes it that natural
forces and conditions of life so occur and co-operate as to produce the
variety of changes needed?