186 On Aquatic Carnivorous Coleoptera or Dytiscida. 
from one ancestor or a few ancestors. The first of these questions I can answer 
decidedly in the affirmative. That these water beetles have arrived at their present 
condition by a process of gradual modification or evolution, seems possible and con- 
sistent with their structures ; indeed I may go farther and add that there are some 
points of their structure which are not comprehensible on any other hypothesis 
known to me. 
On the second part of the problem I proposed to myself, viz, whether the great 
number of species of Dytiscidee now existing have probably descended from one 
or a few ancestors, I have come to a decidedly negative conclusion. After a care- 
ful study of the various affinities and points of structure, ] cannot consider them as 
indicating genetic community, and I have come to think that it is more probable 
that each species has been evoluted along a distinct and separate line of descent. 
Thus I fancy I see in this mass of twelve hundred species, not a development from 
one ancestor but the results of twelve hundred lines of development. The 
numerous cross-relations between the various aggregates, and the points of re- 
semblance between different species seem when first examined almost irresistibly 
to suggest that they may be accounted for by assuming descent from a common 
ancestor, but more careful study instead of rendering this more probable has 
always had the opposite result. One conclusion, I think, I can state almost 
positively ; it is this, that whatever may prove to be the connexion between existing 
and extinct morphological forms, there is no relationship of an ancestral or genetic 
kind to be traced between actually existing species. This result although negative 
is not without significance, for among these tweive hundred species there are many 
in a later or higher stage of evolution than others, and yet in no case have I been 
able to consider that a lower existing form is ancestral to a higher existing form ; 
the theory of descent from a few ancestors would however lead us to suppose that, 
in some cases at any rate, parental species and descendant species should for a time 
co-exist. Inthe various syntheses forming the third part of this memoir there will 
be found some sketches illustrating the kind of reasoning that has brought me to 
these conclusions. Although quite inclined to agree with Huxley’s remark (“ Manual 
of the Anatomy of Invertebrated Animals,” p. 4) “that the growing tendency to 
mix up etiological speculations with morphological generalisations will, if unchecked, 
throw biology into confusion,” yet I think it must be admitted that if there is to 
be any expression of opinion on etiology, it is well that it should be placed in 
proximity with the observations on which it is founded, for only in such case can 
its true value be appreciated, and I hope in the present instance it will be found 
that the few remarks I have made on these points in no way detract from the 
value of the observations with which they are associated. I will ask also per- 
mission to make now some brief remarks on these etiological problems, my object 
being not to advocate any particular theory, but rather to reiterate the extremely 
difficult nature of these questions, and specially to point out that even if the theory 
