88 
the surface the better they hold. What then is the reason that so 
many of our water-beetles have the upper surface dull and rougher in 
the females ? It might seem to some that a dull rough surface was 
the original form of the various species, and on the face of it there 
certainly seems a great probability that such is the case, the family 
originally springing from some land form which gradually became 
more and more aquatic in its habits, and by the weeding-out process 
of natural selection the smoother specimens Avere preserved and the 
rougher ones destroyed. It is manifestly an advantage for an aquatic 
species to be smooth on account of the less resistance it gives to the Avater 
Avhile swimming, Avhich is an item of considerable importance Avhere 
the habits of the species are carnivorous. This solution at one time 
seemed so obvious to me that I had no doubt it really Avas the 
correct one, in fact, there can be no doubt that the family, in 
common Avith the carnivorous ground beetles Avith Avhich they are 
classed under the heading Adephaga, came originally from a striated 
species, as all the land forms are striated and the aquatic species in 
most cases shoAV traces of striation. We may assume that the aquatic 
species have become smoother and smoother, $ s as usual taking the 
lead, and the fact that in most cases the females are dimorphic some 
being shiny like the males and some being dull, looks as if the females 
Avere gradually folloAving the lead of the males. So far the facts seem 
obvious, and one might confidently predict that a time would come in 
the near future Avhen the Avhole of the Dytiscidae, both males and 
females Avould be smooth, and sexual dimorphism in this family a 
thing of the past. But here comes the difficulty of settling the 
question in this Avay. If the smooth form be the highest development 
of the species, how is it that in tAvo of the highest genera of this 
family the females have made the least progress toAvards the highest 
form, Avhereas in some of the lower forms the females are as shiny as 
the males, nor do they even occasionally throw a dull form AA'hich 
might be classed as a case of reA'ersion, thus showing that the lower 
forms have advanced further in this direction than the higher forms. 
This is a well nigh insuperable objection of itself, since one does not 
go to the highest forms of a family to find vestigial characters better 
developed than in the lower forms of the same family. I think this 
point will be clear to everyone, at all events I am sure no lepidopterist 
Avould try to proA T e the connection between trichoptera and lepidoptera 
by trying to find connecting links between the tAA’o orders among the 
haAvk moths or Bombycids. But another objection even more difficult 
to get over lies in the fact that the sulci in Uytiscus are altogether 
different from the strife of the ground-beetles, and this should certainly 
not be the case if both Avere derived from a common ancestor. Having 
noAV cleared the ground by showing that the females are not dull 
because of the assistance it gives at pairing-time, and also that the 
dull and rough surface is not a A^estigial character, it remains, 
therefore, that the real reason is that the character is of importance 
to the species, and has been recently developed. This is no doubt, 
I think, brought about by the concealment of the species being 
advanced by the breaking up the large smooth surface into a number of 
dark ridges and light furroAvs; this kind of concealment being a AA^ell 
known and very effective one, and in any case a rougher surface, even 
in Avater, is less conspicuous than a shiny one when it is amongst 
weeds and aquatic vegetation. 
