MARINE CRUSTACEANS. 193 
I. VARIETIES! IN THE DECAPOD CRUSTACEANS. 
1. The Nature of Varieties. 
In sorting out a collection of Crabs or Prawns, it is probably the habit of many 
systematists to separate groups of specimens which seem to belong to single species, and 
afterwards to examine each of these groups with more care. Now in many cases such a 
group will prove to be homogeneous, that is, to consist of individuals, differmg from one another, 
it is true, in a greater or less degree, but not separable by their differences into sub- 
groups. Such a group will be regarded as a species. But in other cases careful examination 
of the specimens will reveal amongst them well-marked differences by which they can be 
separated into two or more sub-groups. Further, these sub-groups may either be entirely 
independent of one another, in which case we are dealing with two species confounded at 
first sorting, or they may be connected by specimens? intermediate between them with 
regard to the features by which they are separated, and not otherwise sharply marked off 
from them. Such sub-groups are called varieties. It must, however, be borne in mind that 
the intermediate specimens between varieties are much less numerous than those which 
exhibit the peculiarities of the varieties in a typical development. Otherwise it is only 
possible to say that the species exhibits considerable range of variation, but that definite 
varieties are not established. In my own experience, however, wide variability without the 
formation of definite varieties is rare among Decapods. 
The difference, then, between species and varieties as recognised in systematic Carcinology 
is, or should be, that species are unconnected by intermediate forms, while varieties are united 
into species by the existence of specimens intermediate between them in structure. This is of 
course a purely empirical distinction, but it has the merit of being easily understood, 
workable, not likely to be very differently applied by different workers, and, which is decisive, 
the only one practicable in the present state of our knowledge. With the advantages of 
empiricism, however, the method of distinction outlined above combines one of its disadvan- 
tages, being somewhat too rigid for universal application. It has therefore to be modified 
in practice by the judgment of the worker in certain cases; that is to say, in those m 
which the trend of variation in each of two or more groups of individuals makes it 
probable that a more extensive series of specimens would show complete continuity, though 
there be, for the time, a gap between them. In such cases it is legitimate to unite the 
forms in question as varieties of a single species; though at the time of doing. this the 
uncertainty should be distinctly stated. In point of fact surmises of this sort have repeat- 
edly been justified by later knowledge’. 
1 In this paper the word variety is used to denote an 
assemblage of individuals (as defined below). 4 variation is 
a peculiarity of a single individual (though it is often repeated 
in other individuals, in which case the word may be used 
collectively). Variation—without the indefinite article—is 
the fact of the existence of variations, and variability is the 
name given to the phenomenon of the occurrence of variation 
in-a species or higher group, or between the like parts of an 
individual. The adjective varietal indicates connection with 
a variety. 
2 Hither in the collection or described by former writers 
on the species. 
3 It is, indeed, a mere commonplace that many so-called 
species are mere varieties, to which rank some are degraded 
every year. In the end it will probably be found that every 
true species has definite habits and habitat, which have 
brought about its specific distinctness. It is not, of course, 
on this account any the less important that varieties should 
be named and registered. 
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