1 88 THE EVOLUTION OF SEX. 



merely, and need not further complicate the series. * But the primitive 

 buds which have been carried away give rise asexually to secondary 

 buds; these become sexual, and their fertilized ova give rise to the 

 original ' ' nurse ' ' forms. There are therefore several asexual gen- 

 erations between the sexual, and our formula must run, — 



III. Alternation between Sexual and Degenerate Sexual Reproduction. — 

 The cases we have just noticed are both easier to state and easier to explain 

 than others which are sometimes also included under the vague title of 

 "alternation of generations." The above alternations were between sexual and 

 asexual reproduction ; these must be distinguished, vague as the boundary 

 must be, from alternation between the ordinary sexual process and a degenerate 

 form of the same. 



The adventurous history of some of the flukes {Trematoda) may be taken 

 as a first illustration. The common liver-fluke {EHstomum or Fasciola hepatica) 

 which causes the disastrous "rot" in sheep has a life of vicissitudes. The 

 fertilized ovum gives rise to an embryo, which passes from the sheep, which its 

 sexual parent infested, to the water by the field side. There it leads for a while 

 an active life, knocking against many things, but finally attaching itself to a 

 minute water-snail. Into this it bores, losing its covering of active cilia with 

 change of habit, and becoming much altered into a passive vegetative form 

 known as a sporocyst. Now this sporocyst sometimes divides ; and if this were 

 all, and the results grew up into liver-flukes, we should have the old formula 

 and less loss of sheep. But direct development never occurs, and we may leave 

 the casual division at present out of account. Certain cells within the sporocyst 

 form germs, and these serve in the place of genuine ova. They produce 

 within the body of the sporocyst another brood of what are called Redicr. 

 There may be several generations of them, and the final result is a brood of 

 minute tailed organisms {Cercarice), which leave the water-snails, leave the 

 water even, creep up grass-stems, and encyst themselves. There most wait for 

 death, a few for the attainment of adult sexual life if they chance to be eaten by 

 a sheep. The somewhat complex story may be written in lines : — 



The fertilized ovum gives rise to an aquatic embryo (I). 



This enters a water-snail, and becomes a "sporocyst." 



(The sporocyst may divide.) 



Within the sporocyst, cells develop into " Rediee" (II). 



There may be several generations of rediae (III., IV). 



The last generation ( Cercarier) may become adult sexual liver-flukes (V). 



This can not be accurately ranked as parallel to what occurs among the 

 above-mentioned tunicates, for the rediae arise from precocious reproductive 

 cells. These can not be called ova, and there is no fertilization, but yet the 



