26o BREEDING 



theory and the Mendelian one are, of course, expressed 

 in different terms, but in their essential features they 

 are identical. And, I think, it is very difficult to 

 regard this circumstance as a mere coincidence. It 

 is not likely that either of them is closely in touch 

 with actuality, but that up to a certain point they 

 agree is an indication that a clue has been found. 



We have now to deal with pieces of evidence, very 

 different from one another and from those already 

 considered, which strongly support the view that 

 one sex is a hybrid, or, to use a word of wider 

 signification, bipotential, and the other pure, or 

 unipotential, but point to the female as pure, or 

 unipotential and the male as hybrid or bipotential. 



The first of these indications is derived from 

 Mr. Geoffrey Smith's observations on crabs in the 

 Bay of Naples. The female of this species of crab is 

 characterised by the breadth of her abdomen and 

 the presence, on it, of long filamentous appendages 

 to which the eggs are attached when they are laid, 

 and by the delicacy of her pincer-claws. The male 

 is characterised by the narrowness of his abdomen, 

 the possession of a pair of hard copulatory appen- 

 dages instead of a number of flexible filamentous 

 ones, and by the much greater size and strength of 

 his pincer-claws. 



This crab is occasionally infected by a parasite, 

 called Sacculina^ which is also a crustacean, though 

 it would never be recognised as such, for it hangs, 

 like a full wallet, from the body of its host, whilst 

 inside it consists of a ramifying system of fibrils which 



