2o8 THE EVOLUTION OF SEX. 



mainly in the number of generations through which the parthenogenesis 

 may be ccMitinued. 



Where the starting-point is a fertilised ovum. 



P = parthenogenetic female, producing a parthenogenetic 



ovum, from which arise other parthenogenetic forms, 



or eventually 

 S=^male and female. 



§ 7. Alternation of Different Scxnal Generations. — The rhythm may 

 be followed in yet a higher scale. In a very few cases there is an alter- 

 nation between two different sexual generations. Thus one of the thread- 

 worms {Leptodera appendiciilata) found in the snail gives rise, by the or- 

 dinary sexual process, to a different form, which leads a free life, and 

 subsequently gives origin to the parasite. In both generations the sexes 

 are distinct. More remarkable still is the history of another nematode 

 {Angiostoniiim nigrovenosiim)^ found in the lung of the frog. It is physio- 

 logically hermaphrodite, though its organ is ovary-like ; its eggs are 

 fertilised by its own sperms, which mature tirst ; the progeny become sexual 

 — males and females — in the earth, and their offspring return to the frog, 

 where they become hermaphrodites. Another example of alternation of 

 sexual generations is found in one of the threadworms which occur in man 

 { Rhabdonenia strongyloides). 



§ 8. Occ7irrenee of these Alternations in Animals. — From sponges to 

 tunicates such alternations occur. Beyond the latter, unless we wish to be 

 very subtle, they cease. It is necessary to be clear about the fact that 

 asexual and sexual reproduction may occur together in the same form. 

 The common hydra gives off buds in an entirely asexual way, but it is also 

 a sexual animal, with male and female organs. There may be periods of 

 vegetative growth and climacterics of sexuality in the same organism, with- 

 out any alternation of generations. 



It is possible that the term alternation of generations may be applied 

 to some of the phenomena observed in the Protozoa. Thus Brandt main- 

 tains that all the colonial radiolarians, known as Sphcerozoa, form on the 

 one hand isospores, which are all equal and apparently parthenogenetic, 

 and on the other hand anisospores, which are large and small, — in fact, 

 sexually dimorphic. He believes — though the fact cannot be called 

 demonstrated — that two unequal anisospores unite to form a double cell, a 

 fertilised unit, which will produce isospores again, and these the normal 

 colony. The generation of these sphairozoa is further complicated {a) by 

 division of the colonies, {l>) by division of the individuals of young vegetative 

 colonies, and {c) by the formation of special " extra-capsular " reproductive 

 bodies in young colonies. 



The history of the common fresh-water sponge [Spongilla), as told by 



