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THE AMERICAN NATURALIST [Vol. XLVII 



would not mean that the mating of brother and sister was 

 not inbreeding, or that it was the equivalent of self- 

 fertilization. 



The position of East and Hayes, as indicated in the 

 quotations given, seems to me to amount to a proposition 

 to throw away entirely as meaningless all kinship ele- 

 ments in genetic descent. Is this not a bit premature? 

 It is true that "a cousin marriage may be a wide cross, 

 it may be very narrow." But does this fact justify from 

 the standpoint of experimental science, and in the present 

 state of knowledge, the generalization of the preceding 

 sentence: "the statement must be made very emphatic 

 that investigations such as studies of cousin marriages 

 in the human race amount to nothing?" Is not the real 

 task of science here to investigate and compare cousin 

 marriages which are wide crosses and cousin mar- 

 riages which are narrow ones? In other words, there 

 would appear to be two variables here, not one. I can 

 not regard the results of East and Hayes, important as 

 they are, as justifying the closure of a field of experi- 

 mental science in which as yet very little has been done. 



Eeturning now to the main problem it may be inquired : 

 What, if any, is the relation of the coefficients of inbreed- 

 ing to zygotic constitution? Do the coefficients tell us 

 anything regarding this matter? A little consideration 

 shows that they do. The successive coefficients of in- 

 breeding indicate the rate and degree to which the pos- 

 sible number of different heredity unit factors present in 

 the ancestry is subsequently reduced as a result of in- 

 breeding. They give no indication, as has already ap- 

 peared, of the condition in which the remaining factors 

 are present (i. e., whether in homozygous or heterozygous 

 condition). The meaning here will be clear if a concrete 

 example is considered. When one brother and sister 

 mating is made 50 per cent, of the maximum possible 

 number of different ancestors is eliminated. It is at least 

 readily conceivable, if indeed it can not be said to be 

 highly probable, that no two individuals among higher 

 animals and plants are exactly alike in zygotic constitu- 



