306 THE EVOLUTION OF THE METAZOA 



an extraordinarily important stage in the evolution of 

 life (Unicellularia, according to Rotmiiller; Eunucleata, ac- 

 cording to Moskovskij). The growth in size of an individual 

 is necessarily limited by such a monokaryonik state. This limit 

 was overcome several times during the subsequent phylo- 

 genetic evolution which, in agreement with a generally valid 

 regularity, did not involve all the lines of evolution that had 

 developed from one and the same origin. In the meantime a 

 new bifurcation emerged ; plants and animals. This bi- 

 furcation has also taken place in several lines of evolution. 

 Complex and therefore easily enlarged bionts had evolved 

 along the line which w^as pursued in the evolution of plants 

 by way of the so-called formation of colonies; its reason was 

 a strongly developed cell wall. The phrase "formation of 

 colonies," however, which is so frequently used is not com- 

 pletely adequate. The monocellular individuals of one and 

 the same species can also unite into a biological whole even 

 if they belong to different colonies. Coenobia and poly- 

 cellular algae are regularly developed from descendants of 

 a zygote, more rarely from a vegetative cell (a spore). The 

 products of division do not scatter, they rather remain together 

 and develop into parts of a new unit which are organically 

 connected, they develop into a complex individual. This 

 process can be considered as a parallel to the process of the 

 polymerization. The daughter cells are not only all powerful 

 they are even primarily destined to develop into independent 

 individuals. They lose part of their "sovereignty" by becom- 

 ing members of a coenobium or of a colonial union, they 

 become subordinated to the new unit or entity. This leads 

 via practice, to a heterogeneity of originally equal and equival- 

 ent cells, to their differentiation. It can be observed above 

 all in the development of the generative and of the somatic 

 cells. Tissues, organs, and systems of organs are also developed 

 in this way. The freely moving way of life is relinquished 

 quite early by all the plant bionts (with a few exceptions, 

 Volvocales !). They become permanently sessile beings which 



