832 On Aquatic Carnivorous Coleoptera or Dytiscidce. 



by stating the probability that in the Triassic or an earlier epoch the land of the 

 Australian continent was continuous with that of the Arctogreal 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 httle 

 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 Pelobii 

 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 sjjecies 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 Dytiscidfe 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 Dytiscidae. 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 Dytiscidae : and the more I have become acquainted with the 



