THE EVOLUTION OF PROTOZOA. 307 



■the latter class, such as Astasia, are more sluggish than others 

 of equal size with only one flagellum (e. g. Peranema). Nor does 

 the possession of the extra flagellum in the forms in which it is 

 used for trophic purposes seem to have given these forms any 

 advantage in the struggle for existence over their less well- 

 furnished relatives, judging the case by the only admissible 

 standard, that of preferential survival. Indeed, it is difficult to 

 see how it could, since at present the food-supply is quite 

 plentiful enough for all, and there is no reason to believe that it 

 has been otherwise since very ancient times ; bacteria and 

 decaying vegetable matter were probably abundant in the 

 Palaeozoic seas. In this case too, therefore, we seem to be 

 driven to admit that natural selection is not competent to explain 

 the facts. 



3. Evolution of the Ciliata. 



The history of this step in evolution, like that of the one just 

 discussed, is one of the further perfecting of what may be termed 

 the " tropho-motor system " ; that is, of the system of cell- 

 organs concerned with feeding and movement. The general lines 

 •of development seem fairly clear. The ancestor of the Ciliates 

 must have been Flagellates related to the existing Tetramitina. 

 It probably possessed several flagella placed anteriorly, and was 

 somewhat pear-shaped. The flagella seem to have multiplied 

 by antimeric fission in such a way as to become arranged more 

 •or less around the mouth ; at the same time they probably 

 became shorter and stiffer, since a number of long flexible 

 flagella would become entangled and neutralise each other's 

 efforts. It then must have much resembled the existing 

 Lophomonas, a linking form for whose preservation biologists 

 have to thank that somewhat objectionable insect, the cock- 

 roach, of which it is a parasite. Further progress consisted 

 in the increased shortening of the flagella to the condition of 

 short rods, and the reduction of their " musculature" to a con- 

 tractile strip on one side, which, in conjunction with an elastic 

 strip on the other side, caused a rhythmic sweep instead of the 

 sinuous flexion of a flagellum. The organism was now a 

 Ciliate, and must have borne a close resemblance to the very 

 primitive peritrichous form Strombidium. 



