164 THE EVOLUTION OF MAN. 



conditions of aggregation are the same in states as in 

 many-celled organisms. 



Finally, we must mention an eighth and last function 

 of organic development, viz. coalescence, or concrescence. 

 As yet, this has been but little noticed, nor is it very 

 striking; yet it is of real importance in certain processes. 

 Coalescence consists in this, that two or more individuals 

 which were originally separate afterwards combine and 

 blend into one individual. We may regard the process 

 of sexual generation as a coalescence of two cells. We also 

 often find a similar coalescence of cells in other processes of 

 evolution. Those tissues of the animal body which dis- 

 charge the highest functions, viz. the muscular tissue, or 

 flesh, which is concerned in locomotion, and the nervous 

 tissue which performs the functions of sensation, will, and 

 thought, consist in great part of coalescent cells. But not 

 only cells, or individuals of the first order, but also 

 organs, or individuals of the second order, coalesce very 

 freely in the process of Ontogeny into a compound 

 formation. Even independent organisms may coalesce, as 

 is very often the case, e.g. in the Sponges. The process 

 of coalescence (often also called conjugation or copulation), 

 is in a certain sense the opposite process to that of propaga- 

 tion. In the latter two or more new individuals arise 

 from one, while in the former one individual results from 

 several. As a general rule, this individual possesses a higher 

 function than that of the two units from the coalescence of 

 which it sprang. 



In reviewing for a moment the different vital activities 

 of the organism which we have here enumerated as the 

 e&sential functions of evolution— as the true formative forces 



