ALTERNATION OF GENERATION^ 'IN, S 



budding form by the egg-producing form, and so on 

 indefinitely. Sars a few years later showed that the 

 common jelly-fish Aurelia also propagates by eggs and 

 buds alternately. Here the familiar swimming- disks, 

 which are of two sexes, produce eggs from which 

 locomotive larva issue. The larva at length settles 

 down and takes a Hydra-like form. It pushes upwards 

 an ascending column, which divides transversely and 

 forms a pile of slices, each destined to become a free, 

 sexual Aurelia. The alternation of generations may be 

 regarded as resulting from the introduction of budding 

 into the early stage of a life-history which culminates 

 in sexual reproduction, much as if a caterpillar were to 

 divide repeatedly and form more caterpillars, each of 

 which ultimately became a moth. The case which has 

 been given as an illustration actually occurs in nature. 

 A parasitic caterpillar, that of Encyrtus, divides while 

 still an embryo, so that one egg produces several 

 moths. 1 Many other cases of alternation have since 

 been found among animals, and it seems to be the rule 

 among plants. 



Alternation of generations may be complicated by 

 association with transformation, by the omission of 

 stages usual in the class, and by budding-out from one 

 part instead of from the whole body. In particular 

 cases the complication becomes so great that biological 

 language breaks down under it. Such terms as 

 generation, individual, organ, larva, adult, cannot 

 always be used consistently without either being 

 strained or artificially limited. 



1 The same process of " embryonic fission " occurs in other 

 animals also, one of which is a mammal (Praopus). 



