188 On Aquatic Carnivorous Colcoptcra or Dytiscidce. 



the hypothesis to any great extent. I think his argument might be carried a step 

 farther, and it might be pointed out that whereas the acceptation of the theory of 

 evohition in no way diminishes the importance of the physiological differences 

 between species, it does on the other hand very much detract from the value of the 

 morphological agreement between them ; at any rate it is I think clear that the 

 physiological distinctions between different species may be real and permanent even 

 though the structural characters by which they are represented in our classifications 

 shall prove to have been fluctuating or temporary. 



We know from paleontology that the totality of the organisms of the present day 

 is very different from what it was formerly as regards the morphological structure ; 

 but we di not finrl traces of any facts that would lead us to believe that the earlier 

 organisms were not isolated into separate species just as the present ones are. 

 While the excessive resemblance between the ova of different animals coupled with 

 the fact that these similar ova develope into totally distinct organisms, leads us to 

 believe that distinctions of a most profound character may have been present in the 

 earlier and simpler organisms even though the imperfection of our means of investi- 

 gation would not allow us to assert this from the study of the organisms themselves. 

 Thus it seems to be a perfectly credible theory that the isolation of species from one 

 another is indicative of deej) seated distinctionsthatmayhaveexistedpriorto the more 

 conspicuousmorphological differences ; and itappears probable that though the species 

 of a late geological epoch may be the descendants of differently structured animals of 

 a former epoch yet the physiological distinctions may have been more permanent, 

 or in other words that the lines of descent have been perfectly distinct. It will at 

 any rate be incumbent on those who advocate community of descent to show how 

 and when the physiological distinctions became established, and at present, so far 

 as I know, there is no evidence of the beginning of such distinctions ; we are not 

 aware of any process by which a group of individuals fertile inter se, becomes divided 

 into two or more distinct, mutually infei'tile groups. But those who maintain the 

 community of descent tacitly assert that such a jirocess has been the order of 

 Nature. 



There seems good reason for supposing that the physiological distinctions 

 between species are correlative with molecular distinctions that we do not at 

 present comprehend ; and if so we may hope that the advance of the science of 

 pure molecular physics will help to solve these problems. But if we are not 

 content to wait patiently for such a period, and at present allow our imaginations 

 to attempt to penetrate very far back into the darkness of the past, we may well 

 ask those who maintain the descent of organisms from one or a few ancestors, 

 what reason have they for supposing — what satisfaction can there be in believing 

 — that only one or two or a few primitive organisms or germs existed ? 



Surely, if the passage from matter in a state of inorganic arrangement to a state 

 of more complex organic arrangement took place naturally — under the influence of 



