204 THE EVOLUTION OF SEX. 



§ 3. Alterjiation between Sextcal 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 histoiy of some of the flukes {Trematodd) may be taken 

 as a first illustration. The common liver-fluke {Distomiun or Fasciola 

 hepatica) which causes the disastrous "rot" in sheep has a life of vicis- 

 situdes. The fertilised ovum gives rise to an embrj-o, 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 Redu?. There may be 

 several generations of them, and the final result is a brood of minute tailed 

 organisms {CercaricT), which leave the water-snails, leave the water even, 

 creep up grass stems, and encyst themselves. There most w^ait for death, 

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

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



The fertilised 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 ^^ jRedicd'' (2). 



There may be several generations of rediae (3, 4). 



The last generation [Cercan'ce] may become adult sexual liver-flukes (5). 



This cannot be accurately ranked as parallel to what occurs among 

 the above-mentioned tunicates, for the rediee arise from precocious repro- 

 ductive cells. These cannot be called ova, and there is no fertilisation, but 

 yet the process is not one of division, or of budding. It is a degenerate 

 process of parthenogenetic reproduction in early life. The facts may be 

 again summed up in a formula, which does not take account of the occa- 

 sional division of the "sporocyst." 



A, asexual lar\'-ae ; S, sexual fluke ; the upper circles represent 

 the special germ cells ; fertilised ova at the base. 



The germ-cells, which behave like ova, and yet do not rise to that level, 

 appear sometimes in a central mass within the asexual individual, some- 



