VII. SEXUAL REPRODUCTION AND REJUVENESCENCE 269 



cytoplasm into which the new structure is to be spread. For this 

 reason sex had to be differentiated ; the particles, active gametes, 

 which are liberated from the one are called male gametes, or sperms, 

 and produced in much greater numbers than larger female gametes, 

 or egg cells, which are provided with great quantities of cytoplasm. 

 In the case of the structural recombination of two viruses above 

 cited, there are host cells in which the new structure can multiply ; 

 without host cells new structure, of course, cannot multiply. 



If a weaker individual is devoured completely, its structure will 

 be of no use, but if a part of the structure can be intermixed with 

 the structure of the stronger individual, some character of the weaker 

 one can remain in the combined structure to make the newly formed 

 individual more fitted for existence ; the part of the structure which 

 can remain in the newly formed individual must be strong and so 

 may give rise to a distinct character. Therefore, the individuals of 

 assimilase or primeval primary organisms, which had succeeded in 

 obtaining the faculty for the sexual reproduction would accomplish 

 much more rapid progress in their evolution than those lacking the 

 faculty, and consequently would become the fittest with the result 

 that at present higher organisms without exception perform sexual 

 reproduction. In short, the first aim of the conjugation may be the 

 rejuvenation, but the yield of new structures may be another import- 

 ant aim. 



It is true, however, that sexual reproduction is very troublesome, 

 and the easiest and simplest way must be the reproduction by fission 

 without sexual conjugation. This must be the reason why certain 

 organisms choose the alternation of generation, or metagenesis, chara- 

 cteristic of plants , also occurring among a few animals. It involves 

 an alternation of a sexually-reproducing with an asexually-reproducing 

 generation ; in this latter generation vigorous multiplication occurs 

 because of its easiness, while in the former generation rejuvenation 

 may take place with best chances to form new individuals more fitted 

 for existence, although the multiplication in this generation is not so 

 easily done as in the asexual generation. 



Such a mode of multiplication must have been highly developed 

 presumably because it was the best way to become the fittest for 

 some plants and animals, whereas it must have been inconvenient for 

 some other organisms exhibiting certain other forms and functions 

 and therefore these organisms achieve sexual reproduction only. 



Microorganisms, such as paramecia or other certain protozoa, 

 which multiply usually by binary fission and have not yet acquired 

 the undoubted, distinct mode of metagenesis, appear to accomplish 

 rejuvenation by performing now and then an irregular conjugation. 



