On Aquatic Carnivorous Coleoptera or Dytiscida. 187 
of evolution be true, community of descent is not rendered probable thereby. I 
have already stated that the existence of taxonomical series does not really lend 
any support to the theory of a few ancestors, and I would now add to this that 
similarity of structure as to some particular point also fails to establish the pro- 
bability of community of descent ; for if one hypothetical ancestor can have 
developed a point of structure, equally can two or more have done so. 
Semper has recently pointed out that “ every character which can be regarded 
as a true sign of large groups of animal forms, may be ultimately traced to the 
stage at which it first appeared, and where it was a character of adaptation ” 
(“ Animal Life,” notes p. 407). This is perfectly correct, and to it may be added the 
fact that if we look at some character that is now probably in process of adaptation, 
we find that the adaptation is going on not in one favoured species, but in a number 
of allied species. For instance, it is the rule in the Dytiscidz that the meso- and 
metasterna are connected together in the central line ofthe body; a considerable 
number of the group Hydroporini form however an exception to this rule, and one 
of these genera, Deronectes, differs only from Hydroporus by the fact that this 
connection is wanting in the former while it has been attained in the latter genus ; 
but a study of the species of Deronectes seems to show that the connection in 
question although not at present existing 1s probably being gained by many of the 
species if not actually by all of them. Now if a structure be acquired simultaneously 
by a number of distinct species, it is clear that similarity of structure does not 
indicate community of descent. Again if I am right in supposing the species of 
Deronectes are acquiring this structure by the want of which they are solely dis- 
tinguished from Hydroporus, it is plain that these two distinct genera are, so far as 
can be seen, in process of becoming one : the real difference between them is in fact 
one of time—Hydroporus has gained a particular structure before Deronectes has 
done so. Such facts appear to me gravely opposed to the @ priori probability of 
descent from a few ancestors. Butif the resemblances between animals do not 
justify the theory, there remains on the other hand the important fact that the 
isolation of species from one another is gravely opposed to its probability. Huxley 
has pointed out in his essay On the origin of species (“‘ Lay Sermons, Addresses and 
Reviews’), that the term species expresses two different sets of facts ; first, a set of 
morphological facts, or the agreement of series of individuals in points of structure ; 
and second, the physiological inductions that animals consist of groups of individuals 
that are fertile enter se, but who do not produce fertile offspring when crossed with 
members of other groups; and in the same essay he has stated that when we look 
at the facts from the point of view of the morphological agreement between indi- 
viduals then the theory of community of descent is possible or probable, while if we 
bear in mind the physiological isolation of species then the theory is unsatisfactory. 
The arguments of Huxley in this essay appear to me very good, except that as I have 
already pointed out the morphological facts do not when carefully considered support 
TRANS. ROY. DUB. SOC., N.S., VOL. II. 25 
