I94 THE EVOLUTION OF SEX. 



Among type-worms, the strict alternation of generations in some of the 

 marine chstopods (sylids), the more complicated phenomena of so many 

 trematodes, the sexual rhythms of that peculiar threadworm Angiostomum, 

 have been already discussed. It is necessary, however, to state the case for 

 tapeworms, which are usually included among the examples of alternation of 

 generations. The usual view is, that the embryo of a tapeworm develops into 

 an asexual bladder-worm, which asexuallv buds off a "head," or more than 

 one. Such a "head," passing to another host, buds off asexually the chain of 

 reproductive joints or sexual individuals which constitute a tapeworm. Asexual 

 bladder-worm, asexual "head," and sexual joints, form the series. That there 

 is a genuine alternation of generation is believed by some authorities, but there 

 are emphatic difficulties against this supposition, except in the occasional 

 occurrence of a bladder-worm with several "heads," each of which may 

 develop into a tapeworm. The case is well stated by Hatchett Jackson in his 

 monumental edition of Rolleston's "Forms of Animal Life," and we accept his 

 verdict that there is really one individual throughout, except when asexual 

 multiplication of heads occurs. The tapeworm, on this view, is an adult sexual 

 bladder-worm, and the joints are only highly individualized segments. 



Of the parthenogenetic cycles in crustaceans and insects, the juvenile repro- 

 duction of some of the latter, and the true alternation of generations in some 

 tunicates, enough has already been said. 



Von Jhering is responsible for starting the paradox that in higher animals a 

 mother may bring forth her grandchildren. He refers to the case of the hyaena- 

 like carnivore Praopus, where a single ovum gives rise to eight embryos, which 

 are thus in a pedantic sense grandchildren ! The frequent occurrence of twins 

 in all groups, the remarkable case of an earth-worm (Lumbricus trapczoidcs) in 

 which a double embryo is constant, and the morphological resemblance of 

 polar globules to abortive germs, led Von Jhering to maintain that the origin of 

 multiple embryos from a single ovum is the primitive and normal condition, and 

 that the development of only one is secondary and adaptive. The data are 

 hardly sufficient for such a striking conclusion. 



IX. Occurrence of Alternations in Plants.— In the lower 

 plants, algae and fungi, an alternation between spore-producing and 

 truly sexual generations is frequent. In mosses and ferns it is almost 

 constant, and yet more marked. Occasionally either spore-formation or 

 sex-cell formation may be suppressed, and the life-history thus simpli- 

 fied. In a few of the higher plants both are exceptionally suppressed, 

 and we have thus a reversion to a purely vegetable process, just as if 

 a hydra went on giving off daughter-buds without ever becoming 

 sexual. In the flowering-plants, what corresponds to the sexual 

 generation of a fern is much reduced; it has come to remain continuous 

 with the vegetative asexual generation, on which it has reacted in 

 subtle physiological influence. Just as in the higher animals, alter- 

 nation of generations finds at most only a rudimentary expression. 



X. Heredity in Alternating Generations. — The problem of 

 the relative constancy of inheritance is now in part solved by the 

 theory of germinal continuity. The ovum which develops into an 



