186 On Aquatic Carnivorous Coleoptera or Dytiscidoe. 



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 mc. 



On the second part of the problem I proposed to myself, viz, whether the great 

 number of species of Dytiscidae now existing have probably descended from one 

 or a few ancestors, 1 have come to a decidedly negative conclusion. After a care- 

 ful study of the various affinities and points of structure, I 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 tliis mass of twelve hundred species, not a development from 

 one ancestor but the results of twelve hundred lines of development. The 

 numerous cross-7-elations 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 tlie 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 twelve 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. In the 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 setiological speculations Avith morphological generalisations will, if unchecked, 

 throw biology into confusion," yet 1 think it must be admitted that if there is ta 

 be any expression of opinion on aetiology, it is well that it should be plueed in 

 proximity with the observations on which it is founded, for only in such ca-e 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 setiological 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 



