in the Decapod Crustacea. 337 



cuUna may recover and become the progenitor of a numerous 

 family. 



It may also appear paradoxical to say that it is advanta- 

 geous to the crab to thoroughly protect its parasite. Never- 

 theless it is easy to convince one's self, by examining a crab 

 bearing a SaccuUna, that every excitation of the parasite 

 pioduces a contraction the recoil of which is very painful to 

 the crab. The more the SaccuUna is sheltered the less the 

 crab must experience pangs resulting from the contraction 

 of the Rhizocephalan when irritated by external objects. 

 And, further, if the SaccuUna happens to be severely wounded, 

 it will die and become decomposed on the spot, often causing 

 the death of the crab, whose viscera are affected by the putre- 

 faction of the roots of its enemy. On the other hand, if the 

 SaccuUna grows old, suitably protected, it has still an exist- 

 ence much shorter than that of its host, and when it dies its 

 roots in time undergo a sort of dry degenerescence which 

 does not seem at all to place the life of the crab in danger. 



A iiriori therefore we cannot entirely reject the influence 

 of natural selection in the transmission of the capacity on the 

 part of the male crabs of acquiring certain modifications ; but 

 we nevertheless think that this explanation must be discarded 

 upon considerations of comparative physiology. 



It has long been known that the castration of the males of 

 mammals and birds results in giving to the animals subjected 

 to it the secondary sexual characters of the female sex. Per- 

 haps it would be more correct to say that, in these cases, as 

 in that under consideration, castration prevents the develop- 

 ment of the male sexual characters. However this may be, 

 in geldings and capons the sterility is complete and definitive. 

 Now the modifications which they present are completely of 

 the same nature as those which we have indicated among the 

 Crustacea. We have therefore to find an explanation which 

 may apply to all the cases. 



2. We might seek this explanation in what Darwin has 

 called " latent characters." Of these it is precisely the 

 secondary sexual characters which furnish the best example. 

 " In every female," says Darwin *, " all the secondary male 

 characters, and in every male all the secondary female cha- 

 racters, apparently exist in a latent state, ready to be evolved 

 under certain conditions. It is well known that a large num- 

 ber of female birds, such as fowls, various pheasants, partridges, 

 peahens, ducks, &c., when old or diseased, or when operated 

 on, partly assume the secondary male characters of their 



* * Variation of Animals and Plants,' vol. ii. p. 51. 



