ig2 



THE E VOL UTION OF SEX. 



rather more differentiated than those in the young flukes, but the close 

 parallelism is indubitable. Except that there is for a while no fertilization, the 

 process can hardly be called asexual. The formula may be expressed in a 

 gentle curve : — 



Fig. 72. — Where the starting-point is as before a fertilized ovum ; 

 L = prematurely reproductive larva ; 

 ps = precocious parthenogenetic ' ' pseudova " ; 

 S = adult sexual male and female organism. 



Somewhat different is the curious case of Gyrodactylus, a trematode 

 parasitic on fresh-water fishes, where three generations are fouad inclosed, one 

 within the other, in a fashion which recalls the fancies of the preformationists. 

 In this case, however, it seems likely that internal fertilization really occurs. 



VI. Alternation of Parthenogenesis and Ordinary Sexual Reproduction. — 

 In our gradual ascent, we now reach the frequent alternation of parthenogenesis 

 and ordinary sexual reproduction. The special cells which develop without 

 fertilization are now genuine parthenogenetic ova, and the organisms which 

 produce them are adults, not juveniles. The formula; will differ mainly in the 

 number of generations through which the parthenogenesis may be continued. 



Fig. 73. — Where the starting-point is a fertilized ovum. 



P = parthenogenetic, female, producing a parthenogenetic 

 ovum, from which arise other parthenogenetic forma, 

 or eventually 



S = Male and female. 



VII. Alternation of Different Sexual Generations. — The rhythm may be 

 followed in yet a higher scale. In a very few cases there is an alternation 

 between two different sexual generations. Thus one of the threadworms 

 {Leptodera appendiculata) found in the snail gives rise, by the ordinary 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 (Angiostomum nigrove- 

 nosum), found in the lung of the frog. It is physiologically hermaphrodite, 

 though its organ is ovary-like ; its eggs are fertilized by its own sperms, which 

 mature first ; 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 {Rhabdoncma strongyloides). 



