THE CONSERVATION OF LIFE 243 



But we must not anticipate. "Let us therefore return to the 

 unicellular organisms. Among the Vorticellae we saw numerous 

 forms whose mode of reproduction in so far resembled the re- 

 production of the ovum, as with them division did not lead to 

 a complete separation of the newly formed individuals. We saw 

 that from one animal gradually proceeded many-headed polyp- 

 stocks, the single individuals of which remained somewhat loosely 

 connected by means of plasmic stalks. But though this con- 

 nection in the Vorticellae is slight and only the individual cells 

 are independent of each other, it represents nevertheless the 

 first step which leads from the protozoon to the multicellular 

 organisms. It is true that the most important difference between 

 them and the higher organisms, the division of labour between 

 the single parts of the community, is as yet absent, unless we 

 conceive the formation of two kinds of sexual cells to be its 

 beginning. For this reason the advantage of this close associa- 

 tion is difficult to understand, and yet a reason must exist, for 

 how else can the habit of making only an incomplete division 

 have been fixed by heredity? 



It is interesting to observe that other species of protozoans, 

 which generally lead a lonely life, occasionally proceed to the 

 formation of a colony. This is another proof that such union, 

 even without division of labour, must be useful, though we are 

 not always able to perceive its advantage. Thus the parasitic 

 gregarines, which have long been known as intestinal parasites 

 of lower animals, and the Peridinia which live in enormous 

 swarms in European ponds and waters, are frequently observed 

 to exist as long 'chains of dozens of individuals. 



Most striking, however, because apparently giving proof of 

 intelligence, is the system of commensalism, as observed in 

 particular in the brilliant-coloured Heliozoa. If two, three, or 

 more of these animalcules accidentally meet near a prey which a 

 single Heliozoon would be unable to overpower we notice that 

 their pseudopodia suddenly fuse on contact. Like hunters who 

 surround a head of game the Heliozoa form round their prey a 

 circle which gradually closes. The rigid central filaments which 

 ordinarily give to the pseudopodia their firm shape become 

 rudimentary, the pseudopodia shorter, and the circle more and 

 more narrow until finally all the bodies have become fused into 



