ALTERNATION OF GENERATIONS. 207 



The difference between these two alternations has been as often pointed 

 out as it has been ignored. The former is called true alternation of 

 generations (or metagenesis) ; the latter is called by zoologists, in reference 

 to flukes for instance, heterogamy. Comparisons between the alternations 

 in plants and animals have seldom recognised the distinction. 



Let it be recognised, however, and we can readily proceed to more 

 complicated cases where the two are combined. Returning to the liver- 

 fluke and others like it, we find that the sporocyst sometimes multiplies in 

 a genuinely asexual fashion — without the intervention of precocious ova, 

 special reproductive cells, germs, or spores, call them what you will — 

 by direct division or budding. For such cases the formula must be 

 modified as follows : — 



The complication is not serious. It is simply that, before the multipli- 

 cation by special cells sets in, there may be more than one (A', A") entirely 

 asexual (and not merely sexless) generation. 



§ 5. Alternation of Juvenile Parthenogenetic Reproduction with the 

 Adult Sexual Process — We have already noted the curious precocity of some 

 midge larvae, which reproduce while still young. Cells within the body, 

 apparently precocious ova, develop parthenogenetically into larvae, which 

 prey upon the mother larva, eventually kill her and leave her, only themselves 

 to become in turn similar victims of precocity. This may continue for a 

 series of generations, with continuous decrease in the size of the reproduc- 

 tive cells, till finally true sexuality and adult life is attained. The repro- 

 ductive cells here are rather more differentiated than those in the young 

 flukes, but the close parallelism is indubitable. Except that there is for a 

 while no fertilisation, the process can hardly be called asexual. The 

 formula may be expressed in a gentle curve : — • 



Where the starting point is as before a fertilised ovum ; 

 L = prematurely reproductive larva ; 

 ps = precocious parthenogenetic "pseudova" ; 

 S = adult sexual male or female organism. 



Somewhat different is the curious case of Gyrodactylus, a trematode 

 parasitic on fresh-water fishes, where three generations are found enclosed, 

 one within the other, in a fashion which recalls the fancies of the preforma- 

 tionists. In this case, however, it seems likely that internal fertilisation 

 really occurs. 



§ 6. Alternation of Parthenogenesis and Ordinary Sexual Reproduction. 

 —In our gradual ascent, we now reach the frequent alternation of partheno- 

 genesis and ordinary sexual reproduction. The special cells which develop 

 without fertilisation are now genuine parthenogenetic ova, and the organisms 

 which produce them are adults, not juveniles. The formula; will differ 



