D.—ZOOLOGY 75 
suitable for the simple feet of the lower forms and for the elaborate 
suckers of the higher ones, but there is one point which may be urged as 
showing the necessity for the evolution of a more prehensile foot and that 
is the increased size of individuals in the higher groups. A size census 
of the Dytiscids shows that the Hydroporine include the smallest species, 
the Colymbetine, those of intermediate size and Dytiscine the largest, 
each group overlapping its neighbour. ‘Therefore those who regard this 
evolution as the result of natural selection will suggest that the inefficient 
individuals have been constantly weeded out and that the more efficient 
have bred more efficient, and so on. When we remember that both in 
Colymbetine and Dytiscine species with a more simple and a more 
complex arrangement of suckers are to be found side by side, each stage 
of complexity being entirely efficient for its purpose, we may be justified 
in wondering where any elimination of the unfit takes place. 
Another character of the male feet, to which it seems impossible to 
attach any vital importance, is the form of the claws at the apex, which 
are usually two in number. The claws of the front feet may be alike, 
fine, gently curved and tapering to a point, or they may be sharply curved 
at the base or they may differ from one another. ‘The males of neighbour- 
ing species may be distinguishable by them, as we have seen, and the 
differences in these claws seem to be unrelated to other characters. In 
the Hydroporine, the inner or anterior claw of certain species has an 
extension on the inner face, which might be described as a tooth, but 
only odd species possess it and not all the species in a genus or section 
of a genus. In Hydroporus, for instance, seven out of the thirty-six British 
species show this ‘ tooth,’ but they are distributed in four sections of the 
genus. In Agabus again, the distribution of species with a toothed claw 
follows no recognised relationship. Sharp made twenty-three sections in 
this genus and of these, so far as I can find, only six or seven contain 
tooth-clawed species ; in some sections all the species having a tooth, in 
others only odd species. 
In practically all female Dytiscids the anterior and middle feet claws 
retain the normal curve and tapering form and suggest usefulness, but 
it is difficult to ascribe any utility to the modified claws of many males. 
The toothed claws are definitely prevented from performing any function 
of gripping, while in other cases the shape or size also makes impossible 
any such function, and yet in some genera the form of these male claws 
can be used for systematic purposes. 
The claws of the hind feet, on the other hand, show a definite line of 
evolution, as did the tarsal suckers of the male forefeet. In the Noterine 
the hind legs end in a pair of delicate, gently-curved and pointed claws, 
but in the Laccophiline reduction has taken place and although a number 
of authors have described two claws, in Laccophilus at least, only one 
exists. In the Complicati the arrangement seems to be constant for most 
of the genera, but each seems to have had its own idea as to these claws. 
The genera can be grouped as they have two equal or two unequal claws, 
but although those genera usually recognised as lower in the scale all 
come into the ‘equal’ group and most of the higher genera into the 
