§32 On Aquatic Carnivorous Coleoptera or Dytiscide. 
by stating the probability that in the Triassic or an earlier epoch the land of the 
Australian continent was continuous with that of the Arctogzeal continent. If that 
be the case and if the Pelobii have remained almost without evolution since that 
period, it is clear that we must go back in the world’s history to a period of 
enormous antiquity in order to find a time when Pelobii shall have been little 
different in their structure from their nearest allies. In actual fact the study of 
Pelobius does not suggest any belief in the theory of descent from ancestors 
common to it and other beetles, but rather tends to convince us of its absolute 
isolation. 
If, however, we decline to imagine that the European and Australian Pelobit 
have remained nearly the same in structure since their separation from one another 
in the Triassic epoch, and prefer to believe that they have evoluted much since their 
separation, we must in that case admit that extensive processes of organic evolution 
may be carried on in the most distant parts of the world and may extend over a 
large portion of the world’s history and yet result in the production of almost iden- 
tical structures ; an admission which would render nearly worthless all that has 
been written on the subject of geographical distribution, based on the theory of 
descent from common ancestors. 
I myself consider that on the whole this second alternative is perhaps the more 
probable ; I can see no sufficient reason for supposing that the process of evolution 
in these creatures has been entirely checked since the Triassic epoch, but I can well 
suppose that it has gone on, though probably very slowly, since that period, and 
that the Australian and European forms have not diverged much from one another 
because the laws of growth or evolution are radically the same in all organisms, 
and because the environment limiting and regulating those laws has been practically 
nearly the same in both, notwithstanding their wide geographical separation. 
In order to derive full advantage from the study of Pelobius it is not at all necessary 
to believe in the theory of the descent of distinct species from a common ancestor ; 
the only hypothesis necessary to an understanding of all the facts are first that the 
laws of growth (evolution being a form of growth) are fundamentally the same in 
all organisms, and second that similarity of environment limits and regulates 
these laws so as to produce conformably similar results on similar organizations. 
Whatever we may think on these points one thing is quite clear, viz., that though 
Pelobius shows differences from Dytiscidee of such a nature that those who support 
the theory of descent would point to it as a proof of the correctness of their theory, 
yet there is not the least reason for believing it to stand in an ancestral position to 
any known Dytiscide. The only relation of this kind that could be suggested. 
with the least approach to credibility is between it and Colpius ; but an examination 
shows that though in certain highly important points the approximation is very 
great between the two, in other points they are as distinct and widely separated 
as are any two Dytiscide: and the more I have become acquainted with the 
