ASPECTS OF KINETIC EVOLUTION 365 



DIVERSITY IN LENGTHS OF CONJUGATE PERIODS. 



The patterns of longitudinal sections of the networks of de- 

 scent of different species are determined by the longevity of the 

 individual organisms. In popular language it might be said 

 that the generations of some species overlap while those of other 

 species do not. Many species, both of animals and of plants, 

 are strictly annual. All of the adults die in the fall, and the 

 species exists in the winter only in the form of eggs, spores or 

 seeds. These hatch or germinate in the spring and all the new 

 individuals grow to a simultaneous sexual maturity, interbreed, 

 reproduce and die. All the members of the species are in nearly 

 the same condition at the same time and the figure of descent 

 is simple and regular. 



A few species, such as the bamboos among the plants, pre- 

 serve this complete simultaneity, although living through a con- 

 siderable series of years. Flowers and fruits may be produced 

 only at rare intervals of two or three decades. All the plants 

 of the species reproduce at the same time and then die. But in 

 nearly all groups the lengthening of the life of the individual 

 organism means the overlapping of the generations and the 

 simultaneous existence of many different forms or stages of the 

 species. 



Such a statement is not adequate, however, for a scientific 

 description of the complexities of overlapping descent ; for the 

 word generation has been used with a great diversity of mean- 

 ings. In the lowest unicellular organisms each independent 

 cell-individual is a generation. In the next stage, where the 

 cells are joined into simple and relatively undifferentiated struc- 

 tures, the word generation may well denote the interval between 

 two successive conjugations, or rather the structure which is 

 built up between the ending of one conjugation and the ending of 

 the next. But even this definition fails us as we go higher in 

 the scale of existence and find plants and animals which build 

 two or more organic structures between successive conjugations. 



In some cases there is a succession of two kinds of cellular 

 structures, one structure being built up before the formation 

 of the sex-cells, before conjugation commences, and another 

 structure after conjugation has commenced. The former is 



