cS oe 

196 L. A. BORRADAILE. 
may be included that brought about by limitation of an organism to certain zones of depth, 
or bathymetrical isolation. 
ii. “Physiological,” due to a greater or less degree of infertility accompanying structural 
differences between allied organisms. 
iv. “Sexual,” due to a preference by the members of a species or variety for mating 
with their own kind. 
Local variation is, of course, far from unknown in the Decapoda, but of the class of 
varieties we are now dealing with one of the most marked characteristics is their independ- 
ence of geographical limitations. In fact it may generally be safely foretold of a Decapod 
crustacean that, if one variety be taken im a given locality, any others that may be known 
will probably also be found there, if only the collection be large enough. In the great 
majority of cases, if not in all, the varieties of the Decapoda are not known to affect 
special habits or habitats. Of course it is also true that very little is known about the 
habits of the group at all: but this does not justify us in assuming that non-local varieties 
are adapted to special habits. In a few cases of varietal species which I have examined 
“im the field” I have been unable to find any evidence of such a phenomenon. We have 
seen that, in the case of Thalamita admeta, there is no sign either of varietal habits or 
of varietal habitats as far as our present knowledge goes. An even better example is 
T. exetastica Alc., whose three varieties have been taken on every kind of bottom and in 
varying positions with regard to the passages of the reef. To take another instance, from 
a different group of Decapods, the porcelain crab Petrolisthes lamarcki has four varieties which 
show no divergence either in habits or in habitat. Regarding bathymetrical isolation there is 
again no evidence of varietal separation. We have seen that the case of T. admeta is 
somewhat doubtful. That of T. ezetastica is beyond question in this respect, all the varieties 
having been taken within a few fathoms of one another in the Maldives. Yet T. exetastica 
is a species with a well-marked specific bathymetrical range, the thirteen recorded captures 
(including that made by the “Investigator”) being all below 26 (or possibly below 30) 
fathoms. 
As to physiological and sexual isolation in the Decapoda there is no evidence. 
2. There is no evidence of adaptation of varieties to special varietal habits or habitats. 
This, which is really an entirely different question from that of isolation, resolves itself into 
a restatement of the evidence just given. For, so long as hardly anything is known of 
the adaptation of species by their structure to their specific habitats or habits, it is idle 
to argue that there is no evidence of the utility of varietal characters. But we do know, 
or at least are beginning to know, that every species has its specific habitat and _ habits, 
to which we conclude, from the few cases that we understand, that its specific characters 
are adapted. It is therefore quite pertinent to argue that, unless varieties can be shown 
to have definite habitats and habits, their varietal characters cannot be assumed to be 
adapted to such circumstances. And it is of just this phenomenon—the existence of varietal 
habitats and habits—that there is at present no evidence. 
3. There is no clear evidence of intermediate stages in the formation of varietal from 
homogeneous species. There are plenty of species in which it is impossible to pick out two 
1 Coutiére [Les Alpheidae, Paris, 1899] seems to have oor two Alpheid varieties, but at present there is always a 
found at Jibuti examples of habitative isolation among one doubt as to the interpretation put on the term variety. 
