24 HISTORY OF CRUSTACEA. Chap. IV. 



of tlie olfactory filaments or of the chelae would disappear 

 from the lists, and two sharply defined forms, the best 

 smellers and the best claspers, would remain as the sole 

 adversaries. At the present day the contest seems to 

 have been decided in favour of the latter, as they occur 

 in greatly preponderating numbers, perhaps a hundred 

 of them to one smeller. 



To return to Bronn's objection. When he says that 

 " for the support of the Darwinian theory, and in order 

 to explain why many species do not coalesce by means 

 of intermediate forms, he would gladly discover some 

 external or internal principle which should compel the 

 variations of each species to advance in one direction, 

 instead of merely permitting them in all directions," we 

 may, in this as in many other cases, find such a principle 

 in the fact that actually only a few directions stand open 

 in which the variations are at the same time improve- 

 ments, and in which therefore they can accumulate and 

 become fixed ; whilst in all others, being either indifferent 

 or injurious, they will go as lightly as they come. 



The occurrence of two kinds of males in the same 

 species may perhaps not be a very rare phenomenon 

 in animals in which the males differ wddely from 

 the females in structure. But only in those which 

 can be procured in sufficient abundance, will it be 

 possible to arrive at a conviction that we have not 

 before us either two different species, or animals of 

 different ages. From my own observation, although 

 not very extensive, I can give a second example. It 

 relates to a shore-hopper (Orchestia). The animal 



