85 
some specimens of D. marginalis and watched them till two paired, 
and found that the beetle put one of the anterior legs over the edge 
of the thorax and pressed the disc on the under side, while the other 
disc was placed on the upper surface, he then watched another pair, 
which did practically the same thing, except that the opposite leg in 
this case was pressed to the underside of the thorax, while the leg 
which, in the other case was on the underside, was in this case on 
top; this can hardly be the regular method of pairing, but whether 
or no, it was perfectly clear from the position that the bristles on the 
anterior and intermediate femora of the male would fit into the 
sulci of the female elytra, and by careful comparison I found 
that the bristles on the femora of the males are better developed 
than in the females, so we see that not only do the ordinary facts fit 
in with the idea that the sulci of the female are a new and necessary 
development to the species, but we also find out the new fact that the 
male is also developing a character to suit the altered condition, and 
which at present is too slight to have been noticed at all had not the 
theory suggested the search for it. 
We are now, I think, in a position to take this not as a possible 
theory, but as a probable fact, and Ave can see from this point some¬ 
thing of the development of this character in the family, and from that 
something of the past history of the family of genera. 
We have six species of Dytiscus occurring in Britain and four of 
these species have two forms of the female, one shiny and smooth 
like the male and one with sulcate elytra; of these four Canon FoAvler 
in his manual of British Coleoptera only refers to one species, namely, 
D. circumcinctus, showing any intermediate form betAveen the sulcate 
and the shiny form, and Ave have then the three gradations : 
D. circumcinctus with sulcate, shiny and intermediate forms. 
D. marginalis 
D. lapponicus 
D. circumflexus 
D. punctulatus 
D. dimidiatus 
Avith sulcate and shiny, but no intermediate forms. 
| Avith sulcate form only. 
As the sulcate form has been shoAvn to be the highest development, 
and taking the British specimens by themselves (as I have not the 
time or material to discuss the matter as it should be with the Avhole 
genera, including exotics), Avhat we learn is that probably D. circum¬ 
cinctus was the species nearest allied to the ancestor which, after the 
sulci were developed, was the progenitor of the genus, and the other 
five species came from the same stock at a later period, but of the fiA-e 
only tAvo have so far perfected themselves on this point, as neA T er to 
revert to the ancestral form, Avhereas the other three still occasionally 
revert to the smooth form, and all this tells us that the formation of 
the various species has been a rapid and recent one, since so many 
revert, and it is also clear that the splitting up of the species was 
before the character Avas firmly fixed, and consequently the species 
themselves were recently developed. This view is corroborated by the 
bristles on the femora of the male not being very markedly more 
developed than in the females. 
By this means a light is throAvn on the posterior coxal de\*elopment 
in the genus. The pointed coxal processes I was inclined to think 
the more highly developed than the blunt ones, but since they 
are 
