336 SEX IN MICROORGANISMS 



to the list; nor is it claimed that the generalization we have in our 

 turn offered has yet received 'final form,' if that phrase indeed be 

 ever permissible in an evolving science, except when applied to what 

 is altogether extinct." Like many other authors, Geddes and Thom- 

 son held that sex differences were primarily metabolic in character. 



Just how many additional "theories" about sex have been offered 

 since 1889 is not known, but only three will be outlined here and 

 they are based largely on conditions in the Protozoa, the group with 

 which I am most familiar. The first may be designated the "hunger" 

 theory; the second may be referred to as the "chance fusion" theory; 

 and the third consists of the ideas suggested by Cleveland and based 

 on his studies of sex phenomena in the animal flagellates living in 

 the gut of the wood-feeding roach, Cryptocercus piinctulatus. 



THE "HUNGER" THEORY 



Dangeard employed a metabolic concept in discussing the pos- 

 sible evolution of "sex." He repeatedly expressed himself on this 

 subject but made a somewhat more extended statement in 1913-14. 

 He based his theory to a considerable extent on the life history of the 

 chlamydomonad flagellates; in this group rapidly ensuing fissions pro- 

 duce "spores" (zoospores) which, as gametes, fuse together to 

 produce zygotes. 



Dangeard stated that, with simple bipartition, the intermediate 

 period of nutrition suffices to maintain nutritive equilibrium. How- 

 ever, the nutritive condition finds itself unbalanced at the end of a 

 series of sporulating divisions. The "spores" are enfeebled, famished, 

 incapable of development. This diminution of vital energy of ordi- 

 nary "spores," under the influence of insufficient nutrition, is, accord- 

 ing to him, the cause which has provoked, in the course of evolution, 

 the appearance of sexuality. 



In his general conclusions, Dangeard expressed himself essentially 

 as follows: (1) Sexual reproduction has had for its cause a nutritive 

 origin, the results of successive bipartitions without intermediate 

 periods of nutrition. (2) The gametes are like ordinary "spores" 

 (zoospores) but are weak and incapable of continuing their own 

 development. (3) Sexual reproduction has been derived from asexual 

 reproduction. (4) The attraction which unites the gametes is of the 

 same nature as that which carries an organism toward its prey or 



