On Aquatic Carnivorous Coleoptera or Dytiscidce. 187 



of evolution be true, community of descent is not rendered probable thereby. I 

 have already stated that the existence of taxonoraical seiies 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 ada23tation " 

 ("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 ^Drobably 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 Dytiscidte that the meso- and 

 metasterna are connected together in the central line of the 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 m 

 question although not at present existing is j^i'obably 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 similaiity 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 apjjear to me gravely opposed to the a priori probability of 

 descent from a few ancestors. But if 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 inter 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 jihysiological 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 sujjport 



TRANS. ROY. DL'B. SOC . N.S., VOL. II. 2 C 



