50 ANIMAL LIFE AND HUMAN PBOGRESS 



" dominant." These individuals, $ and 9 , form germ-cells, 

 and in such a way that every germ-cell contains either 

 Core. 



Now when fertilisation between these germ-cells takes 

 place, the following combinations are possible : 



<? O Oc? 



o? $ o? 





and no others, as you can prove by experiment or by 

 mathematics. The combination QO must have short wings, 

 and these individuals, if interbred, only give vestigials. They 

 are the so-called recessives. The individuals Q have only 

 the factor for long wings and they also breed true. But the 

 individuals 0O and O0 have both factors. They are long- 

 winged because . when present, always asserts itself. But 

 when interbred they always behave like the cross in Fl, that 

 is, they always produce offspring in the proportion 3 long : 

 1 vestigial. 



The experiment is simply a repetition on an animal, the 

 fruit-fly, of Mendel's original experiment with green and 

 yellow peas. Mutatis mutandis the result arrived at is 

 identical. 



Hundreds of such experiments have been made with 

 animals and plants, and always with the same result. 



Though time forbids my going into details, in order to meet 

 the objection that the scheme I have just given is only invented 

 to account for the facts, I must tell you that germ-cells are not 

 really simple things. In them, as in all cells, there are little 

 bodies called chromosomes. When the germ-cells get ready for 

 reproduction, or as we say " mature," these little bodies go 

 through curious evolutions and divide and are distributed to 

 male and female germ-cells in such a way that their products 

 actually are sorted out in the manner shown in the scheme. 

 These divisions and sortings out have been studied over and 

 over again and are quite familiar to microscopists. 



There is, therefore, a " mechanism of heredity," and we are 

 dealing with facts, not with assumptions. 



We can further show that, if more than one character i 



