214 AMPHIMIXIS OR ESSENTIAL MEANING OF [XII. 



pensable mechanical conditions are wanting. Hence, in the 

 formation of stocks, amphimixis does not appear in everj' 

 generation of persons, but only periodically in certain genera- 

 tions, and from this follows an alternation between two methods 

 of increase, viz. with and without amphimixis, or, as it is called, 

 an alternation of generation. Many principles come into action 

 in this mode of development, which we cannot stop to consider, 

 above all the gradual development of high individuahsation in 

 the stock, through the differentiation of its persons on the prin- 

 ciple of division of labour, as was expounded many years ago, 

 in a most convincing manner, by Rudolph Leuckart. 



We can furthermore understand why a longer or shorter 

 series of generations elapses before amphimixis becomes asso- 

 ciated with increase : a long interval is the necessary conse- 

 quence of the formation of highly differentiated animal stocks. 



I need hardly say that I do not, by any means, intend to 

 imply that no change in the method of reproduction can have 

 arisen without stock-formation. In the groups of polypes and 

 medusae, among which the above-mentioned alternation of 

 generation is so widely spread, we find species which do not 

 form stocks, and which, after passing through a series of gene- 

 rations by fission or budding, return to the method of sexual 

 reproduction. It is clear that in such cases, the omission of 

 a detailed and dangerous embryogeny, together with the more 

 rapid multiplication which accompanies the omission, has been 

 the efficient cause which has limited amphimixis to certain 

 generations. The fresh-water polype, Hydra, is an example of 

 this. The duration of the ' agamic ' period is so regulated by the 

 external conditions of life that the concentration of the collective 

 predispositions of the species in a single cell, which is associated 

 with amphimixis, is at the same time made use of to form a 

 resting-egg, which carries the species over the unfavourable 

 seasons. 



The adoption of entirely different methods by closely allied 

 animals shows how little the existence and duration of the 

 periods of asexual reproduction have to do with the number of 

 cells composing a single individual. In one and the same group 

 of Hydromedusae we find species with long periods of asexual 

 reproduction side by side with others in which it has entirel}' 

 disappeared, so that every generation proceeds from fertilized 



