182 ANIMAL LIFE AND HUMAN PROGRESS 



give yellows and non-yellows in the proportion 2:1, instead 

 of in the normal 3 : 1 ratio. Extensive breeding experiments 

 led to the conclusion that the homozygous yellow was formed 

 but that it died at an early stage during the embryonic develop- 

 ment. Quite recently this deduction, made from purely genetic 

 work, has been confirmed by several American workers, who 

 have looked for these early embryos in the uterus and found 

 them dead. Factors affecting the viability of the offspring 

 have also been demonstrated by Morgan and his -co-workers 

 with Drosophila, and it is not unlikely that they will play 

 an important part in the future of animal breeding. 



Again, the study of variegated plants has already led us 

 to the conception of individuals made up of components 

 belonging to different races. Plants have been produced of 

 which the inner core is that of the tomato and the outer 

 skin that of the black nightshade or the position of these 

 two species may be reversed, with a resulting plant quite 

 different in form. To-day they are but the curiosities of 

 the laboratory, yet who can say whither they may lead us 

 to-morrow ? 



Nor should mention be omitted of those fascinating studies 

 which treat of the action of external influences upon the germ 

 cells themselves. Hertwig has shown that the sperm of a frog 

 exposed to the influence of radium may lose its function of 

 hereditary transmission and yet retain its power of stimulat- 

 ing the ovum to develop. Parthenogenesis, even among the 

 mammals, is brought within the bounds of possibility. 



For the practical breeder this playing with the germ cell 

 may be fraught with significance. To-day we are limited in 

 our researches by the number of different kinds of germ cells 

 in the world. The making of a new creature means the union 

 of germ cells that have not previously been brought together. 

 We can make use only of such as already exist and can enter 

 into combination with each other. For fresh combinations 

 we must have fresh kinds of germ cells. When we understand 

 more of their architecture we may one day be able to make 

 them, whether through the influence of radium or of some 

 other physical or chemical agency. Out of some wild labora- 



