366 COOK 



built of simple nonconjugate cells, the latter of double or con- 

 jugate cells. The nonconjugate structure corresponds to the 

 " generation" of the simpler types of organization. The con- 

 jugate structure is a new feature intercalated into the previous 

 life-cycle, which it often completely overshadows. The con- 

 jugation period of many organisms, and especially of the highest 

 groups, both of animals and of plants, is now very much 

 longer than the part of their life history which corresponds 

 to a whole generation in the lower groups. For tracing homol- 

 ogies between the higher and the lower groups it is still pos- 

 sible to talk of the period between conjugations as a gener- 

 ation, but most of the generation is now occupied by the 

 conjugation period, the life-time of the double-celled phase of 

 organization. This corresponds merely to the fertilized egg-cell 

 or oospore of the lower algae which do not build up any struc- 

 tures of conjugate cells. 



In other cases, which are properly to be called alternation of 

 generations, the diversity of the two interconjugational forms has 

 been brought about by vegetative propagation, which replaces 

 or supplements the sexual reproduction of the species. Alter- 

 nation of generations, that is, of two forms of organic individuals 

 in the same species, may take place either in the conjugate or 

 in the simple or nonconjugate period of the "generation." 

 Thus in the mosses and liverworts vegetative propagation is fre- 

 quent in the simple-celled phase, while in the ferns and flower- 

 ing plants it appears in the conjugate period. Vegetative pro- 

 pagation is often described as a purely asexual process, but this 

 is not true of the higher plants, since the conjugate phase 

 is wholly a sexual phenomenon, a part of the sexual process of 

 conjugation. 



It may therefore be held that the term generation, as popularly 

 used with reference to the higher plants and animals, does not 

 correspond to what is meant by generations among the lower 

 groups. The period of the life-history which constitutes a gen- 

 eration among the more primitive types of life is so brief as to 

 remain practically unnoticed among the highest. Conversely, 

 the conjugate period which is so short and unimportant as not 

 to complicate the question of generations in the lower groups is 



