90 THE GENETIC AND THE OPERATIVE EVIDENCE 



Giard in 1886. As a result of infection by parasitic Crustacea (e. g., 

 Sacculina), the male crab develops the secondary sexual characters of 

 the female. It has been generally supposed, following Giard, that 

 this result is due to the destruction of the testes of the male by the roots 

 of the parasite that invades the spaces between the organs of the 

 host, and, in the case of the testis, ultimately brings about its partial 

 or complete destruction. Not unnaturally the results here were sup- 

 posed to be parallel to those of castration in vertebrates, and received 

 in fact the name of "parasitic castration." More recently Geoffrey 

 Smith has studied this phenomenon in the crab Inachus, infected by 

 the parasite Sacculina, and has reached the conclusion that the change 

 is not due to injury or to destruction of the testes, but to a change in 

 the metabolism of the crab brought about by the parasite. 



Taking Geoffrey Smith's case of Inachus-Sacculina as typical, the 

 changes brought about are as follows : The parasites attach themselves 

 to the young crabs before the external secondary sexual differences have 

 appeared. In the females, the effect is to cause them to develop pre- 

 maturely the distinctively female characters. In the male, on the 

 other hand, the narrow abdomen of the male changes after a molt into 

 the broad abdomen of the female, which also develops ovigerous 

 appendages on its ventral surface like those of the female in every 

 detail. The larger claw of the male changes into that of the female, 

 which is different in form as well as in size. Some years ago I ven- 

 tured to raise the question as to whether these effects on the male 

 might not be interpreted as retention of the juvenile characters rather 

 than development of the female characters in the male. This might 

 appear more especially the case in the somewhat more juvenile shape 

 of the anterior abdominal appendages and possibly also in the shape 

 of the broader abdomen; but Smith has later shown that the results 

 can not be interpreted as juvenile, for when the changed organs are 

 examined in detail they are found to differ from the same organs in 

 the juvenile condition, and to be identical with those of the adult 

 female. I think, therefore, that we must accept this interpretation of 

 Giard and of Smith as correct. But Smith goes further and believes 

 that the effects may be carried so far that eggs develop in the old 

 testes; in other words, that the testis changes to an ovary. It seems 

 to me that the evidence to support this last point should be much 

 stronger than that advanced by Smith before we can accept this 

 interpretation, for we lack the essential control for this evidence. In 

 only a single case were eggs found — in the testis of a male that had been 

 infected, but from which the parasite had fallen off, and which was 

 presumably recovering from the effects of its presence. Now, it is 

 known that in the testes of some male animals a few eggs may occa- 

 sionally be found where there is no suspicion that the animal has 

 changed its sex. In some Crustacea, in scorpions, and in insects, 

 isolated instances of this kind have been found. Abnormal division 



