MARINE CRUSTACEANS. 197 
well-characterised non-local varieties, and again there is a certain proportion in which two 
or more such varieties are seen. But in all these latter, so far as I have met with them, 
the varieties are sundered by well-marked gaps. That is to say, in sorting fifty specimens 
of two varieties, there would not, in my experience, be more than two or three whose 
position would be doubtful. This is, of course, merely negative evidence, but it would not 
be right to assume that positive evidence in the opposite direction exists till it be 
forthcoming". 
In view of the facts just stated, it is, I think, clear that there are considerable 
difficulties in the way of explaining the origin of varieties in the Decapoda by means of 
natural selection. Time and research may remove these difficulties, but it is equally likely 
that they will not. It will be well, therefore, to consider shortly the alternatives. A 
Lamarckian view of the question, as compared with the Darwinian, is equally difficult to 
reconcile with the want of any correlation of varietal characters with the environment, which 
is implied by the absence of geographical and habitative isolation. There is left, if both 
these fail us, but one alternative possible. Like smaller variations, those which characterise 
varieties would appear to be produced by intrinsic forces alone, of whose equilibrium they 
are perhaps the outcome* But questions so momentous are not to be decided by an 
examination of one group only, nor by any but those who have made variation a study. 
In their hands may be left the question “Are varieties the outcome of inward or of outward 
forces ; does the solution of the problem of their origin lie with Physiology or with Natural 
History?” 
Turning now to an examination of the ways in which varieties might become species, 
and assuming that natural selection is the transforming agent, we may consider in what 
manner it would act. There are two conditions under which natural selection can come 
into play upon a varietal species. Either the environment may alter, or the species may 
spread into a new area where the environment is different from that under which it has 
hitherto been living. 
1 Throughout this article statistics have been avoided, as 
any attempt to draw exact conclusions in a particular case 
from less than, say, a thousand specimens would be hazardous. 
But, to take the first example to hand, out of 36 specimens 
of the type variety of Thalamita admeta collected in the 
Maldives, two only show characters which caused hesitation 
as to whether they should be placed with the remaining 34 
or with examples of var. savignyi. These two were of middle 
size. One small specimen showed some resemblance to var. 
edwardsi, but its position could noi be said to be doubiful. 
? This alternative, if, as seems probable, it coincide with 
Bateson’s theory of Discontinuous Variation, will receive 
from that fact an advocacy which will make it worth careful 
consideration. The chief point in which it amplifies 
Mr Bateson’s theory would seem to be that it is now sug- 
gested that discontinuous variations may coincide, and so 
produce entities still more like species than assemblages of 
individuals separated on one character only, and that there 
is no evidence of this coincidence being brought about by 
natural selection, but on the other hand a possibility of its 
being due to intrinsic causes (correlation). The same cause, 
for instance, which produces ridges in one part of the body 
may, under the different conditions of growth prevailing in 
another region, make tubercles or granulations, and the 
swelling of a surface may lessen automatically the relative 
area of granulation or the height of the granules. Of course 
yery many varieties are separated by one character only, or at 
all events only one such has yet been detected in them. The 
origin of such a form is a simpler problem than that of 
one characterised by two or more special features, and is 
that which has been chiefly discussed above. 
No doubt eases exist of discontinuous variations which do 
not coincide. But in that case one would probably be more 
conspicuous than the others, and its use for systematic 
purposes would cause the others to be overlooked. An 
examination of a key to the varieties of a species will some- 
times reveal such characters of secondary prominence by the 
fact that it is possible to rearrange the key in two or more 
ways. In these cases it is easy to see that the key by no 
méans necessarily represents genetic affinities, but merely 
classifies possible combinations of characters. But I am not 
here concerned to uphold Mr Bateson’s theory, and shall 
enter no farther into a discussion of this point, which concerns 
variation, not varieties. 
