MARINE CRUSTACEANS. 195 
reef, as indicated by the nearness or remoteness of passages. In four dredgings var. granosi- 
mana was taken on a bottom of muddy sand, on another with coarse sand and small rubble, 
on coarse sand with rubble, and on a hard (rocky) bottom. In two of these dredgings weed 
was present, in two it was not. The number of captures was not enough to make it pos- 
sible to come to any conclusion as to the effect of the neighbourhood of passages on the 
distribution of this variety, but in a similar case—that of the varieties of TY. ewetastica— 
it is quite clear that this factor is without influence. Var. savignyi was only taken twice, 
but admeta and intermedia, which were found four and six times respectively, showed no 
greater tendency to restriction to special environments than did granosimana. 
2. The Relation between Varieties and Species. 
Among the Decapod Crustaceans, then, species are paralleled by other assemblages of 
individuals, known as varieties, which differ from them neither in the nature of their 
characteristic features nor in the magnitude of these, but only in being connected into 
groups of two or more by the existence of intermediate individuals. It is hardly possible 
to resist the conclusion that, in many cases at least, species have arisen from such definite 
varieties as these by the extinction of the intermediate individuals. And it will be inter- 
esting to consider of what nature and origin varieties may be, and what processes may turn 
them into species. 
The orthodox explanation of the origin of varieties such as those we are now dealing 
with would, no doubt, be that they are produced by natural selection of the variations 
(generally smaller im degree than those which characterise varieties) which are found within 
the limits of homogeneous species and varieties. Now there can, of course, be no reasonable 
doubt that natural selection is at work among the Decapoda, and it is probable that in 
some cases (as, for instance, in that which Professor Weldon has investigated in Carcinides 
maenas) it brings about transformations of species by accumulating small variations. This 
is especially likely to be taking place where a variety is locally restricted’. But im the 
case of varieties not so restricted, as we are now using the term, it is very difficult to 
accept the same view, and that for three reasons. 
1. There is no evidence of isolation such as is presupposed in the evolution of two or 
more varieties simultaneously from a single species. And it should be observed that this 
is the problem now before us, not the replacement of an outworn type of the species under 
stress of uniformly changing environment over its whole range. 
Isolation in the organic world may be of four kinds: 
i. “ Geographical.” 
u. “Habitative’—a name which I propose to give to the separation of allied organisms 
which, living in the same locality, are nevertheless separated by their habits of life—as 
one prawn will hide among the branches of corals and another in a sponge, one crab burrow 
just below high-water mark and another just above, and so on. With this kind of isolation 
1 To these cases the name of ‘‘ subspecies” might well be Potamon, and perhaps Piluwmnus, where large numbers of 
confined, leaving that of “variety” for non-local forms such species, differing in small points from one another, are of 
as those now under consideration. The ultimate result of local distribution. 
such a process is probably to be seen in genera like Sesarma, 
