292 PROTOZOA AND METAZOA CHAP. 



It is perhaps hardly necessary to caution the reader 

 against the error that there is any connection between 

 the theory of heterogenesis and that of organic evolution. 



It might be said if, as naturalists tell us, dogs are 

 descended from wolves and jackals, and birds from 

 reptiles, why should not, for instance, thread-worms 

 spring from Euglense or Infusoria from Bacteria ? To 

 this it is sufficient to answer that the evolution of one 

 form from another takes place by a series of slow, 

 orderly, progressive changes going on through a long 

 series of generations (p. 222) ; whereas heterogenesis 

 presupposes the casual occurrence of sudden trans- 

 formations in any direction i.e., leading to either a less 

 or a more highly organised form and in the course of 

 a single generation. 1 



Each of the organisms which we have studied in this 

 and the two previous chapters consists of a single cell 

 or in the case of Carchesium and Epistylis of a colony 

 of cells to a large extent independent of one another. 

 They are therefore placed in the lowest primary division 

 of the animal kingdom the phylum Protozoa (p. 220). 

 This phylum is subdivided into a number of classes, 

 examples of certain of which we have examined. Those 

 in which, like the Amoeba, the amoeboid form is pre- 

 dominant constitute the class Rhizopoda : those in which, 

 like the Monads and Euglenae (Flagellata) , the flagellate 

 form is predominant are often included with the ciliated 

 forms (Ciliata) such as Paramcecium, Vorticella, and 

 Opalina in a single class, the Infusoria ; and those in 

 which, like Monocystis, the encysted form is pre- 

 dominant, are known as the Sporozoa. 



1 Apart from such fluctuating or continuous variations, others 

 (mutations] , which may be described as discontinuous, do some- 

 times appear with apparent suddenness, but not to the extent 

 which would be required by the theory of heterogenesis. 



