202 On Aquatic Carnivorous Coleoptera or Dytiscide. 
certain species itis so excessively slight as to be almost inappreciable, and the 
amount of variation to be detected in such cases being likewise very slight, one 
cannot believe in these slight differences of development exercising any appreciable 
effect on the existence of the creature. The sculpture however is most certainly cor- 
relative with sex even in its rudimentary forms; thus in Dytiscus agilis (No. 825) the 
female has the punctuation on the basal portion of the wing-cases more elongate 
than in the male; and in the allied Copelatus atriceps the difference is exhibited 
in a still more rudimentary form: while in the very interesting C. dimorphus 
(No. 827) we have a species displaying true striation in a rudimentary form, and 
in the male in a much more rudimentary manner than in the female. And 
throughout the genus, wherever there is a difference it is that the females have 
the sculpture more developed than the males. We are entitled to believe then 
that whatever the influences may be that have brought about in Copelatus this 
peculiar sculpture, they are influences which have acted at first more strongly on 
the female than on the male, but that continued during a long period such 
disparity has disappeared, or tends to disappear. 
It may be thought that these strie were of assistance in giving holding to 
the claws of the male, and that their direction enabled the male tarsi to arrive at. 
the position most convenient for supporting the insect during the process of 
fertilization, and that the sculpture first gained by the female, was transmitted by 
heredity to the other sex. But these suppositions do not seem to me very 
satisfactory. One does not see why the females should be in the scratched species 
more different from the males, than they are in the striate ones ; for certainly the 
influence of heredity or sexual transference must be acting constantly, and not 
confined to the higher forms of development of the sculpture ; and it is excessively 
doubtful whether the highly developed sculpture is as useful for this sexual 
function as the rudimentary form: for the beautifully perfect straight strie and 
grooves do not seem to me adapted at all for serving a useful purpose of the kind 
above suggested; the twenty-four deep broad parallel strize on the wing-cases of 
Col. sulcipennis can scarcely serve as agents to direct the claws to their requisite 
position, and I am at a loss to see in what other way they would be useful. 
Besides this, it is worthy of remark that in numerous species where the striation 
has attained a great development (similar in each sex) there exists in addition a 
very fine true sexual sculpture, peculiar to the female. The generalization of the 
facts in accordance with any theory of common origin, or of natural selection 
does not seem therefore to be warranted; but on the other hand the presumption 
that each highly developed species has reached its development by passing 
through a series of lower stages similar to such as are found still existing as the 
maximum of development in other species, is perfectly satisfactory and indeed 
iserristible to the imagination; and the conclusion I come to on this subject of 
striation is that if we could gradually subtract from a highly developed species its. 
