222 
invariably recognized by the occurrence in it of a particular 
fossil, it does not matter whether this be a true species or a 
mere variety. Philosophically, however, the system is a bad 
one, and the specific status of a fossil should be determined 
independently of its stratigraphical position. 
Upon the whole, the most feasible explanation of cases such 
as those above recounted, seems to be contained in the admis- 
sion that certain species in certain zoological groups have a 
great range of variability ; that these variable species appear 
under slightly different forms in each successive period or sub- 
period ; and that, though we may, for the sake of convenient 
classification and description, call these by distinct titles, they 
have in reality all been derived from a single primitive type b} r 
some kind of evolution. As before remarked, however, this is 
simply giving a more ample latitude and a broader expansion 
to our conception of the term “ species,” and it does not carry 
with it philosophically the acceptance of the general doctrine of 
evolution. A fortiori it does not bind us to accept any particular 
explanation as to the manner in which these related forms have 
been produced. 
So far as the Darwinian theory is concerned, the facts above 
recounted do not appear to be in any way specially favourable to 
it; since, though they apparently point to some kind of evolution 
having taken place upon a limited scale, they do not show any 
indications of the graduated series of intermediate forms which 
is required by the hypothesis of natural selection, and which 
upon this view must at one time necessarily have existed. If 
Spirifera crispa of the Niagara group, for example, was changed 
by natural selection into S. cycloptera of the Lower Helderberg, 
this into S. tribulis of the Oriskany, and this into S. duodenaria 
of the Corniferous, we ought to find a graduated series of inter- 
mediate forms directly connecting them ; since no ground can 
be alleged why each of the intermediate forms of the series 
should not have had just as long an existence as the four types 
themselves, and should not, therefore, have had just as good a 
chance of being preserved as fossils. Nor do the arguments 
brought forward by Mr. Darwin appear to touch this case, 
since all the forms in question inhabited a single ocean, the 
bottom of which was regularly and slowly subsiding, and in 
which a series of sediments was being accumulated, so far as 
Ave can judge, almost continuously. 
It appears, then, that even in cases such as the preceding, 
Avhich at first sight appear to favour the Darwinian theory, we 
do not find the graduated series of intermediate forms required 
to prove the case. On the contrary, avc find a series of forms 
