ALTERNATION OF GENERATIONS 281 



members the leaves and flowers of a plant ? Clearly be- 

 cause of the conditions which obtain in Hydra. When we 

 see a Hydra bearing one or more buds we do not regard the 

 aggregate as an individual body having members, because we 

 know that, if we watch it long enough, we shall see the buds 

 drop off and lead independent existences. Therefore we 

 ascribe individuality to the buds, even whilst they are 

 attached to the parent form. Nor, if a hydra-bud were 

 accidentally to remain in permanent connection with its 

 parent, should we hesitate to call it an individual and to 

 regard the united Hydra as a compound organism, a sort of 

 Siamese twin made up of two individuals joined together? 

 But if this reasoning holds good for Hydra it clearly holds 

 good for all hydroid colonies on which buds are produced in 

 the same manner as in Hydra. Therefore the members of a 

 hydroid stock, hydranths, blastostyles, and medusre (as well 

 as those other kinds of members which are found in allied 

 genera, such as the defensive persons in Hydractinia), are 

 regarded as individuals, and, since they differ in kind, the 

 colony formed by their union is called Polymorphic. 



Now, reproduction or generation means simply this, that 

 one individual gives rise to another individual. Budding, 

 such as occurs in the Hydromedusae, is therefore an act of 

 reproduction ; and, as it is an asexual form of reproduction, 

 there is an alternation of generations, an indefinite number 

 of asexual generations alternating with a sexual generation, 

 represented in Obelia by the medusa. 



This line of argument is universally adopted by zoologists. 

 It seems unexceptionable, but it has this result, that what 

 they mean by alternation of generations is something quite 

 different to what botanists mean by the same term. In 

 plants the asexual generation or sporophyte produces repro- 

 ductive cells or spores, each of which is capable, without 

 fertilisation, of giving rise to a new organism. This new 

 organism is not the same as the parent sporophyte, but a 

 different form called the gametophyte, producing reproductive 

 cells of two kinds, ova and spermatozoa, which are not by 

 themselves able to give rise to a new organism. But if the 

 two kinds conjugate, the product of their fusion, theoosperm, 

 is capable of reproduction, and gives rise not to the parent 

 gametophyte but to the sporophyte. Thus there is a regular 



