332 SEXUAL SELECTION. Part II. 



distinguished from all other amphipods by the females 

 having "the coxal lamellae of the penultimate pair of 

 " feet produced into hook-like processes, of which the 

 " males lay hold with the hands of the first pair." The 

 development of these hook-like processes probably 

 resulted from those females which were the most 

 securely held during the act of reproduction, having 

 left the largest number of offspring. Another Bra- 

 zilian amphipod (Orchestia Darwinii, fig. 7) is de- 

 scribed by Fritz Miiller, as presenting a case of dimor- 

 phism, like that of Tanais ; for there are two male 

 forms, which differ in the structure of their chelae. 8 As 

 chelae of either shape would certainly have sufficed to 

 hold the female, for both are now used for this purpose, 

 the two male forms probably originated, by some having 

 varied in one manner and some in another ; both forms 

 having derived certain special, but nearly equal advan- 

 tages, from their differently shaped organs. 



It is not known that male crustaceans fisrht together 

 for the possession of the females, but this is probable ; 

 for with most animals when the male is larger than the 

 female, he seems to have acquired his greater size by 

 having conquered during many generations other males. 

 Now Mr. 8pence Bate informs me that in most of the 

 crustacean orders, especially in the highest or the 

 Brachyura, the male is larger than the female; the 

 parasitic genera, however, in which the sexes follow 

 different habits of life, and most of the Entomostraca 

 must be excepted. The chelae of many crustaceans are 

 weapons well adapted for fighting. Thus a Devil-crab 

 (Portunus puher) was seen by a son of Mr. Bate fighting 

 with a Carcinus mmnas, and the latter was soon thrown 

 on its back, and had every limb torn from its body. 



Fritz Miiller, ' Facts and Arguments for Darwin,' 1869, p. 25-28. 



