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 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.