344 Prof. A, Giard on Parasitic Castratron. 



2. As the infested yonn^ male crabs are the only ones 

 which become modified in the female direction, it becomes 

 easy to determine the relative proportion of those which have 

 been infested when adult, which could not hitherto be done at 

 all rigorously *. Our statistics, which unfortunately are only 

 founded upon a restricted number of individuals, prove never- 

 theless that M. Delage was wrong in supposing that infesta- 

 tion in the adult state was quite exceptional. It is, on the 

 contrary, comparatively frequent in Carcinus ma?nas. 



3. As the modification of the external sexual characters is 

 the result of the profound lesion of the genital glands, we 

 must conclude therefrom that the latter already exist at the 

 time of the infestation, or at least that they are in course of 

 formation, which indicates approximately the epoch of the 

 fixation of the parasite. 



VIIT. 



The fact that a jiarasite provokes in its host an abnormal 

 development of organs which protect it at the expense of its 

 AHctim seems at the first glance very exceptional. Neverthe- 

 less we must not see in it any teleological argument, but 

 simply a mutual adaptation which is not without analogy with 

 numerous facts of symbiosis (whether between animals of two 

 different species or between animals and plants), facts which 

 form a series of which the case now under consideration may 

 be regarded as an extreme term. 



The deformations produced in various plants by the Cecido- 

 myice or the Cynipidaj are absolutely phenomena of the same 

 kind. 



An equally curious case is that of the white campion 

 {MeJandryum album) infested by Ustilago antherarum. It is 

 well known that the white campion is normally a dioecious 

 plant. The young flower is hermaphrodite. But upon certain 

 plants the ovaries are aborted ; on others the stamens remain 

 rudimentary. When the parasitic fungus is developed upon 

 a male plant it fructifies in the stamens ; but when it falls 

 upon a female plant it would seem at first that it could not 

 fructify, and this would be so much to the profit of the infested 

 plant. This, however, is not so, for in this case the plant 

 completely develops its rudimentary stamens to enable the 

 parasite to fructify, just as the male StenorhyncJius widens its 

 abdomen to protect the Sacculina Fraissei. Selection acting 

 at once upon the host and upon the parasite has set up a 



* The size of the infested individuals does not furnish a sufficient indi- 

 cation from this point of view, as sexual maturity may be produced in 

 individuals of very different sizes. 



