210 THE EVOLUTION OF SEX. 



heads occurs. The tapeworm, on this view, is an adult sexual bladder- 

 worm, and the joints are only highly individualised segments. 



Of the parthenogenetic cycles in crustaceans and insects, the juvenile 

 reproduction 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 Ijring forth her grandchildren. He refers to the 

 case of the hya^na-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 {Liiiiibricus trapczoidcs) in which a doul)le 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. 



§ 9. Occurrence of Alternations in Plants. — In the lower 

 plants, alga^ 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 simplified. In a few of the higher plants 

 both are exceptionally sup]:)ressed, and we have thus a reversion 

 to a purely vegetative process, just as if a hydra went on giving 

 off daughter-buds without ever becoming sexual. In the 

 flowering })lants, 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, 

 alternation of generations finds at most only a rudimentary 

 expression. 



S^ 10. Heredity in Alter7iating 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 offspring is virtually continuous, either in itself or 

 through its nucleus, with the ovum which gave rise to the 

 parent. A chain of ovum-like cells is only demonstrable in a 

 few cases; but Weismann overcomes this difficulty, by supposing 

 that what really keeps up the protoplasmic tradition or con- 

 tinuity between the parental ovum and the next generation, is a 

 si)ecific and stable portion of the nucleus, — the " germ-plasma." 

 When a medusoid goes off from a hydroid, it carries with it a 

 legacy of this germ-plasma, continuous with that which gave 

 rise to the hydroid. This legacy forms the reproductive 

 elements of the medusoid, which in turn give rise to hydroids. 



