ASPECTS OF KINETIC EVOLUTION 239 



viduals may still be alike ; the organization is still a mere gre- 

 garious association. Later, there may come about a division of 

 labor among the cells, and a corresponding diversification of 

 structure and form. The common pond-scum (Sftirogyra) con- 

 sists of threads formed of cylindrical cells, joined end to end, 

 and all alike in their vegetative and reproductive powers. 

 Another similar organism (CEdogoniuvi) consists, for the most 

 part, of similar chains of equal cells, but these have only vege- 

 tative functions. The power of reproduction has been restricted 

 to two kinds of special sexual cells different from the vegetative 

 cells. 



Advance in the scale of organization not only maintained this 

 distinction between the reproductive and vegetative cells, but 

 continued to increase the numbers and differentiate the struc- 

 tures and functions of the latter, until the immensely complex 

 bodies of the higher plants and animals had been built up. 



The primitive type of cell organization, that which built up 

 the filaments of the lower algae and the vegetative tissues of the 

 liverworts and the mosses was not able, however, to reach the 

 higher possibilities of cellular structure. The cells which com- 

 pose the bodies of the higher fungi have two nuclei, and those 

 of the flowering plants and higher animals have two sets of 

 chromosomes. These double-celled conditions have arisen 

 through a lengthening out of the process of cell-conjugation as 

 it occurred in primitive types like CEdogonium. Instead of 

 conjugating at brief and distant intervals, the cells which com- 

 pose the bodies of the higher plants and animals are in a condi- 

 tion of prolonged conjugation, the cell fusion which begins 

 when the egg-cell is fertilized by the sperm not being completed 

 until after the whole compound cellular structure has been built. 



Several groups of plants have two structural phases, one 

 built of the primitive simple type of cells, the other of the double 

 or sexual type. The moss-spore, when it germinates, first 

 produces a delicate tube like a pond-scum, and the fern-spore a 

 small plate of simple cells, much like a liverwort. These 

 diverse stages or phases of structure of the same organism 

 have usually been described as alternation of generations, but 

 the case is in reality entirely different from the phenomenon of 

 alternation found among animals. 



