122 THE EVOLUTION OF SEX. 



described, although their history has not yet been fully traced. Even 

 in Foraminifcra, as Schlumberger, De la Harpe, and H. B. Brady 

 have shown, a marked dimorphism may occur; and here again the 

 distinction seems to lie between preponderant anabolism and katabol- 

 ism. 



As another illustration, it will be instructive to select the case of 

 volvox. In this colonial organism, which is best regarded as a multi- 

 cellular protist, the component cells are at first all alike. They are 

 united by protoplasmic bridges, and simply form a vegetative colony. 

 In favorable environmental conditions this state of affairs may persist, 



Fig. 3S. — J'olvo.r globator, a colonial Alga or Infusorian, 

 showing the ordinary cells (c) that make up the col- 

 ony (or body), and the special reproductive cells (a, b), 

 both male and female. — After Cohn. 



or be interrupted only by parthenogenetic multiplication. When nutri- 

 tion is checked, however, sexual reproduction makes its appearance, 

 and that in a manner which illustrates most instructively the differen- 

 tiation of the two sets of elements. Some of the cells are seen 

 differentiating at the expense of others, accumulating capital from 

 their neighbors; and if their area of exploitation be sufficiently large, 

 emphatically anabolic cells or ova result; while if their area is reduced 

 by the presence of numerous competitors struggling to become ova, 

 the result is the formation of smaller, less anabolic cells, which become 

 ultimately male, segment into antherozoids, meantime losing their 

 vegetative greenness and becoming yellow. In some species, distinct 



