188 On Aquatic Carnworous Coleoptera or Dytiscide. 
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 
evolutionin 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 do not find traces of any facts that would lead us to believe that the earlier 
organisms were not isolated into separate species just as the preseut 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 deep seated distinctionsthatmay have existed priortothe 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 infertile groups. But those who maintain the 
community of descent tacitly assert that such a process 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 
