FERTILIZATION 213 



complete fusion of two whole organisms at any time other 

 than when they are in the form of single cells; consequently we 

 find vegetative and gamete-forming tissues differentiated side 

 by side, and since these are, as components of a single organism, 

 in a fairly constant environment removed from continuously 

 rejuvenating stimuli, it is the function of the gamete-forming 

 tissues to form single cells which can fuse with cells of other 

 individuals and thus, by altering the composition of the organ- 

 ism, alter its relation to external conditions. And the almost 

 universal association of reproduction and fertilization consid- 

 ered as a rejuvenative process among the Metazoa may have an 

 added significance; the two occur together, not because they 

 are directly related to one another, but because they are both 

 occasioned by the same condition, or rather by the same limi- 

 tation of opportunity, for the complete fusion of multicellular 

 organisms can occur only when these are hi the form of single 

 cells, or gametes. 



Reference to the third possible significance of fertilization 

 may be more brief because of its extremely hypothetical char- 

 acter. The idea that the process of fertilization is primarily 

 related to the phenomena of variation is associated chiefly with 

 the names of Weismann and Oscar Hertwig. The scanty and 

 uncertain character of the evidence here is indicated by the 

 fact that there are two exactly opposed views as to the nature 

 of the relation. Hertwig maintains that the effect of fertiliza- 

 tion is to limit variation within a species, by tending to bring 

 back to the normal, through the process of heredity, the progeny 

 of extreme fluctuations and unusual or abnormal variations 

 (mutations), because the likelihood of their mating with the 

 much more common mediocre or average individuals is so 

 much greater than that of their mating with their likes. Weis- 

 mann, on the other hand, maintains that the effect of syngamy 

 or " amphimixis" is to cause or promote variation, which would 

 result from the new organic combinations in the continued 

 admixture of the gametic nuclei of different individuals. In 

 this way the process of fertilization becomes of great evolu- 

 tionary significance in that it accounts, in part at any rate, for 



