H- I'- I.-IW. V. P. V. I.— 135. 



EVOLUTION OF CELLULAR STRUCTURES. 



INTRODUCTION. 



The value of a new point of view lies in the fact that it permits new 

 relations to be perceived. By means of the kinetic theory of evolution 

 it has become possible to understand that organic development has 

 been carried forward through gradual improvement of the methods of 

 descent rather than by environmental causes. Instead of there beino- 

 a law of heredity which tends to keep the individuals of a species 

 uniform, or exactly alike, the tendency, especially among higher 

 plants and animals, is to maintain, inside the species, a diversity of 

 form and structure, most conspicuously manifested in the phenome- 

 non of sex. 



This intraspecitic diversit}^ is neither accidental nor incidental, but 

 of great physiological and evolutionary importance. The interweav- 

 ing of distinct lines of descent is necessary to sustain the strength and 

 vital efficienc}^ of the individual organisms, and to continue the evo- 

 lutionary progress by which the species adapts itself to changing 

 environments or enters new ones. Interbreeding is as indispensable 

 for the species as for the individual, or even more so, for seedless 

 plants continue their individual existences after the coherence of the 

 specific group has been lost. 



Normal and long-sustained evolutionary progress is not accomplished 

 on single or narrow lines of descent, but is possible only for large 

 companies of interbreeding individuals; that is to say, for species. It 

 is thus no mere accident, but a fundamental necessitv, which brines 

 about the association of organic individuals into species and determines 

 what might be called the specific constitution of living matter. 

 Species are sexual phenomena; they ha^e come where they are only 

 through symbasis; that is, as groups of interbreeding individuals, 

 traveling together along the evohitionary pathway. 



This interpretation of familiar l)iological facts i,s supplemented and 

 confirmed by the study t)f the processes of cell conjugation, which are 

 the means of sym basic interbreeding. Among simple organisms con- 

 jugation is a periodical incident in the multiplication of equal and 

 independent cells. Higher stages of organization were reached by 

 the production within the same species of many kinds of cells and the 

 building of these into large colonies or compound individuals. There 

 was, however, a very earh^ limit to the structures which could be 



9 



