FERTILIZATION 261 



indispensable event in the life history of the Protozoa living under 

 favorable environmental conditions. Certain species have been 

 bred for thousands of generations without conjugation, and, in- 

 deed, without endomixis. Similarly in the Metazoa, both normal 

 and artificial parthenogenesis indicate that the egg itself comprises 

 a mechanism which is capable of initiating and carrying on develop- 

 ment. From this viewpoint, fertilization may be satisfactorily 

 interpreted as a means of insuring under special or unfavorable en- 

 vironmental conditions in unicellular organisms, and under usual 

 conditions in the eggs of multicellular forms, a suitable stimulus 

 which otherwise might be unavailable at the proper time. 



Granting then that fertilization may afford a stimulus to develop- 

 ment, is this its chief significance? Many lines of evidence surely 

 converge toward the view that the opportunities which fertilization 

 affords for changes in the complex of the germ are of paramount 

 importance. Fertilization establishes new diploid groups of heredi- 

 tary characters by combining diverse haploid groups from the two 

 gametes. It makes possible the shuffling of germinal variations so 

 that they are presented in new combinations. It is the pooling of 

 the germinal changes of two lines of descent. Some of the new 

 combinations may more effectually meet — be better adapted to — 

 the exigencies of the environment, and so have a survival value 

 for the organism in the struggle for existence. So whatever the 

 primary meaning of fertilization may be, its importance in estab- 

 lishing the essentially dual nature of every sexually produced 

 organism is settled beyond dispute, and it is the cardinal fact of 

 heredity. No wonder is it that from the lowest to the highest ani- 

 mals provisions are made for a process which multiplies many-fold 

 the opportunities for descent with change. (Fig. 189.) 



In passing, it should be emphasized that provisions to ensure 

 fertilization have had a profound influence on the morphology and 

 physiology of organisms. Sex of the gametes and sex of the indi- 

 vidual body are, of course, radically different, although the latter 

 is indirectly an outcome of the former. The evolution of the gam- 

 etes themselves is relatively simple : from those alike so far as struc- 

 ture is concerned, though physiologically different, to those in 

 which one sex is smaller and more motile and the other larger and 

 usually non-motile. But the sexual evolution of the individual 

 body in the Metazoa presents amazing phenomena. Witness the 



