Unicellular and Multicellular Animals 



described, and indicating that the second has, in all probability, 

 been derived from the first. One of these is shown on Fig. 24. 



There is another interesting fact in connection with Para- 

 moecium. Under natural conditions, 

 division and redivision continue in the 

 ordinary way for a large and indefinite 

 number of generations. But very occa- 

 sionally, a process known as conjugation 

 occurs. Two individuals lay themselves 

 side by side, and partially unite ; they 

 exchange portions of their nuclear sub- 

 stance, and finally separate again, simple ^""P 

 division afterwards proceeding as before. 

 Conjugation, although distinctly different 

 from the ordinary process of sexual re- 

 production, appears to serve the same 

 purpose. Until quite lately its meaning, 

 and that of the process of sexual repro- 

 duction in general, seemed to bid fair to 

 remain a perpetual puzzle to biologists. 

 But at last we seem to be approaching the 

 solution. The characters of a species are 

 determined, it is tolerably certain, by 

 the constitution of the cell nucleus, and 

 accordingly as this varies from one indi- 

 vidual to another, so the characters of 

 the individuals will vary. Now, if simple 

 division were to continue indefinite^, 

 successive generations would be pro- 

 duced on the same plan, and the racial 

 characters would in the main remain 

 constant. But conditions of hfe vary 

 from time to time and from place to place, and the par- 

 ticular type which succeeds best under one set of circum- 

 stances may be ill adapted for another. It is therefore an 

 advantage to a race to be capable of variation. And the process 

 ^ 33 



Fig. 24. — Cercomonas, a form 

 intermediate between the 

 crawling Amoeba type and 

 the free-swimming Para- 

 moecium type. 



