90 



The effect of the Parasites on their hosts. 



2. Sexual differentiation, both primary and secondary, is not entirely due to the action 

 of the sexual formative substance, but rather to the interaction of this substance with the 

 predetermined properties of localized and self-difFerentiating cells. 



3. The effect of parasitic castration is due to a fundamental alteration in the metabolic 

 conditions of the body which reacts on the sexual formative substance, causing simple degen- 

 eration in the female, and degeneration with hermaphroditism either partial or complete in 

 the male. 



4. Ali secondary hermaphroditism, both sporadic and n ormai, may have been imposed 

 solely on the male sex, the female sex being incapable of assuming hermaphroditic cha- 

 racters, either primary or secondary. 



6. Special reference of the phenomena of Parasitic Castration 



to High and Low Dimorphism. 



The principle of High and Low dimorphism has for long been known to affect the 

 sexual development of the males of a number of Arthropods, such as Forjicula and the 

 Lamellicorn beetles. (For literature of the subject see my paper, High and Low Dimor- 

 phism. in: Mitth. Z. Stat. Neapel 17. Bd. 1905 p. 312.) It consists essentially in a quantitative 

 relation between the general size of the males of a species and the degree of development 

 of their secondary sexual characters, such that the larger males exhibit these characters deve- 

 loped to a disproportionately greater degree relatively to their size than the smaller males. 

 Combined with this quantitative ratio we frequently find that the males of these species fall 

 into two more or less sharply defined categories, namely large high males and small low males, 

 the intermediate condition being very sparsely represented. This phenomenon is most clearly 

 to be recognised in animals, like Insects, which do not grow after the attainment of maturity ; 

 in these cases I have termed the type of High and Low Dimorphism which may occur as 

 definitive; but as I have shown in the paper referred to above and as explained on p. 69 

 of this chapter, the essential principle of high and low dimorphism may be exhibited in species 

 which go on growing after the attainment of maturity ; in these cases I have termed the type 

 of dimorphism f acuitati ve. Facultative high and low dimorphism results, in such a species 

 as Inachus scoì-pio, from the fact that the males become mature under the "low" forni while 

 stili quite small, and that before attaining to the "high" condition they have to pass through 

 an intermediate period of active growth in which the sexual development, both primary and 

 secondary, is greatly suppressed and the males then appear as the "middle" males. The high, 

 middle and low types of male in I. scorpio are figured on Piate 7 figs. 6, 7 and 8. The result 

 of this process is that during the breeding season, the breeding males are sharply defined into 

 high and low forms, as in the cases of definitive high and low dimorphism. (I may mention 



