8 4 



THE EVOLUTION OE SEX. 



V. Protozoa and Metazoa. — It has been emphasized above 

 that every multicellular organism, reproduced in the ordinary way, 

 starts from a fertilized ovum, from what may be fairly called a single 

 cell. Sponge, butterfly, bird, and whale, start at the level of the 

 simplest animals or Protozoa, which (with the exception of very loose 

 colonies) remain always unicellular. The simplest organisms leave off 

 where the higher plants and animals begin, that is, as unit masses 

 of living matter. They correspond, in fact, to the reproductive cells 

 of higher animals, and may be called, according to their predominant 

 character, protova and protosperms. A fertilized ovum, as we have 

 seen, proceeds by division to form a " body"; the protozoon remains, 

 with few exceptions, a single cell, in which there is obviously no 

 distinction between reproductive elements and entire organism. 



Fig. 22. — Ophrydium, a colonial infusorian. — From Saville Kent. 



Reference will have to be made to the Protozoa in three connec- 

 tions, which may be here simply noted: — 



(a) In their chief groups, and in the stages of their life-histories, 

 they express phases in the same cell- cycle which recurs in higher 

 forms in the component elements of the body, and in the reproduc- 

 tive cells. The contrast, in other words, between an infusorian and 

 an amoeba, between the ciliated and amceboid stage in the life-history 

 of many forms, is a forecast of the contrast between a ciliated cell and 

 a white blood-corpuscle, between a mobile spermatozoon and a young 

 ovum. That is to say, a predominance of the same protoplasmic pro- 

 cesses is the common explanation of such similarities of form (see 



p. 114). 



(b) It is among the Protozoa that we must presently look, if we 

 hope to understand the origin and import either of "male and female," 

 or of fertilization (see pp. 104, 119). 



