Recent Developments in Heredity and Evolution 19 



expressed by these three groups of forms derived from a 

 splitting hybrid. This means that in a series of genera- 

 tions initiated by a hybrid, approximately one-half of the 

 individuals of each generation will represent the h}'brid 

 mixture, one-fourth of the individuals will represent one 

 of the pure forms that entered into the hybrid, and the 

 remaining fourth will represent the other pure form. 



It should be understood that the use of hybrids in such 

 experimental work is simply a device to secure easy recog- 

 nition of the contributions of each parent to the progeny. 

 For example, if red and yellow races of corn are crossed, it 

 is very simple to recognize the color contribution of each 

 parent to the hybrid progeny, when it would be impossible 

 to separate the contribution of two yellow parents. The 

 inference is, that what is true of hybrids is true of forms 

 produced in the ordinary way, so that laws of heredity 

 obtained from a study of hybrids may be regarded as 

 laws of heredity in general. In one sense, ever}' union of 

 parent forms is hybridizing, for each parent has its own 

 individuality. 



One of the more subtle problems that has arisen in 

 connection with such investigations is the problem of sex 

 determination. In all organisms with sex differentiation, 

 progeny is produced by the fusion of male and female 

 sexual cells, and this progeny develops as distinct male 

 and female individuals. It is one thing to determine the 

 general structure of an organism by some law of heredity, 

 but a very different thing to determine why any individual 

 thus produced is sometimes male and sometimes female. 



The work of today is not resting content with the patient 

 collection of the facts of heredity, with determining ratios 

 as expressions of laws, and with the end results of processes 



