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very closely allied to one another, the two extreme terms of the 
series being conspicuously different, and the intermediate forms 
more or less completely uniting them together ; but, at the 
same time, all the members of the series so far distinct that a 
thoroughly competent and skilled paleontologist describes all 
of them, without hesitation, as distinct and separate species. 
This is not at all what is required for the proof of the Darwinian 
hypothesis, and Mr. Darwin is so fully alive to this that, as we 
have seen, he has devoted much ingenuity to an attempt to 
explain away the absence of the finely gradational forms, which 
upon his theory ought to be found within the limits of each 
great formation. 
So far, therefore, as any actual proof of the Darwinian theory 
of the origin of species is concerned, I believe Palaeontology to 
be at present absolutely silent. The facts of Palaeontology 
point to the operation of some law of evolution, but they do 
not support the special views advanced by Mr. Darwin. Every- 
where we meet with intermediate forms linking together 
different groups ; but these forms are always distinct in them- 
selves and distinct from the types they connect. When we 
look at the “ intercalary” or “ linear” types interposed between 
the great classes of the Reptiles and Birds, for example, 
Compsognatbus, Ichthyornis, Odontopteryx, Archaeopteryx, 
Pterodactylus, and the like, we have a series of distinct struc- 
tural types, which may as a "whole be placed between Reptilia 
and Aves, but which are quite distinct in themselves, and which 
are not connected either wfith one another, or with these two 
classes by any graduated series of transitional forms. Simi- 
larly, Hipparion may be a “ linear type” between Anchitherium 
and Equus ; and in so far as this is probable, it lends support 
to some theory of evolution ; but it does not support the 
Darwinian theory, as we have discovered no intermediate 
forms uniting these very distinct types. The same may be said of 
all, or almost all, of the known “ transitional forms,” which have 
as yet been brought to light by the study of Paleontology. 
In the particular department which we have been investi- 
gating, we have seen that great variability exists in certain 
groups, and that a reasonable probability has been established 
that certain related groups of Brachiopods have descended each 
from a single primordial type. In other words, we have seen 
it to be reasonably probable that certain species are endowed 
with such a plastic organization, that when the surrounding 
conditions change, or in consequence of some unknown and 
inherent law, they undergo modification, and appear in succes- 
sive periods under forms so different, as to have been described 
r 2 
