42 PROBLEMS OF FERTILIZATION 



protozoan groups. Other types of Protozoa exhibit 

 frequently a more definite type of life-cycle, "so that a 

 cell picked out at one phase of the life-cycle is quite a 

 different type of individual from one picked out 

 at another phase" (Calkins, 1916). In these cases, 

 conjugation occupies a definite place in the life-cycle 

 quite comparable to its place in Metazoa; and it may 

 apparently be equally connected with processes of reju- 

 venescence. However, it is clear that rejuvenescence 

 considered as a process of differentiation and relief from 

 "cumulative metabolic differentiations" (Calkins) may 

 be accomplished in Protozoa, as in certain Metazoa, at 

 other times than that of conjugation (fertilization). 



The only result of conjugation or fertilization common 

 to the animal kingdom as a whole is biparental inheri- 

 tance. The association of fertilization with repro- 

 duction or with rejuvenescence is not a universal one, 

 and therefore not a necessary one in the most general 

 sense. In the evolution of the animal kingdom, however, 

 the processes have become more and more inseparably 

 associated in the higher phyla, so that sexual repro- 

 duction becomes the only method for the entire organism, 

 whether of reproduction or of rejuvenescence. 



Sex and fertilization remain for the present ultimate 

 biological categories. We possess innumerable data 

 concerning their manifestations from low to high forms 

 in the animal kingdom; and the researches of recent 

 years have contributed greatly to our understanding of 

 the physiology and genetics of sex, and the morphology 

 and physiology of fertilization. It is only by a con- 

 tinuation of such studies that we can hope to advance 

 farther into the problem of their ultimate significance. 



