228 
the above-mentioned argument we have to assume that this is 
not the first appearance of Productella, but that the genus, or 
sub-genus, had already been in existence elsewhere for an un- 
known but certainly loug period, and had only at this time 
migrated into the American area. If this hypothesis were to 
be accepted, it would doubtless remove, at any rate, some of 
the difficulties of the case, but it would not remove all, and 
there is neither proof nor probability in its favour. If Pro- 
ductella had been in existence elsewhere in Pre-Devonian times, 
it is almost, if not quite, inconceivable that no remains of the 
genus should ever have been found in the Silurian deposits of 
other areas, — deposits which have a very wide extension in space, 
which are enormously rich in Brachiopoda, and which have 
been most diligently searched and examined for fossils. Even 
supposing that at some future time Productella should be 
found to have existed during the Silurian period, the difficulty 
by thisw'ould only be removed a step further back. We should 
still have to believe that this was not really the first appearance 
of the genus, and we should still have to inquire why no 
remains of the genus had been disentombed from the Cam- 
brian deposits. 
When I consider the vast number of cases precisely similar 
in all essential respects to the above, and when I reflect on the 
great extent of uncertain and unexplorable ground traversed 
by the above hypothetical explanation of the facts, I feel com- 
pelled to reject this argument altogether, so far as its general 
application is concerned. The continued introduction of fresh 
types of life, which we know to have gone on since the first 
appearance of organized beings upon the globe, still remains, 
in my opinion, unexplained. It may be that when wc know 
the law under which it has occurred, we shall find that it has 
been in accordance with the Darwinian theory of the origin of 
species. In the meanwhile there is nothing to lead us to sup- 
pose that this will be the case, and it appears to me to con- 
stitute one of the greatest difficulties which this and other 
kindred theories have to surmount, before they can place them- 
selves upon a thoroughly satisfactory basis. 
7. General Conclusions . — As the result of the inquiry in 
which we have been engaged, — an inquiry necessarily extremely 
limited in its range and scope, — the following conclusions may 
be drawn with more or less confidence. And it may be added, 
that though I have only here treated of a single comparatively 
small group of rocks, I know nothing in the entire range of 
palaeontology which would at present confirm with any certainty 
more than is contaiued in these conclusions, so far as these arc 
