Cunningham, Unisexual Inheritance. 3 



characters. The existence of structural differences between the sexes, 

 apart from the essential reproductive organs, is one of the most 

 interesting phenomena in zoology. An adult stag has an enormous 

 pair of branching bony structures attached to his skull, and the female 

 has generally no trace of such organs. The stag is the father of deer 

 both male and female, his male progeny develop antlers like his own, 

 his female progeny show no trace of antlers. Do these familiar facts 

 agree with the hypothesis that only blastogenic variations are here- 

 ditary? We know well enough that some variations are blastogenic, 

 hare lip for example, or the existence of a sixth finger or toe. But 

 these are transmitted indifferently to male or female progeny. 



We have no reason to believe that any kind of selection whether 

 sexual selection or natural selection can explain the limitation of 

 inheritance to one sex. This question has been discussed at length 

 by Darwin in his ,,Descent of Man", 2nd Edition, chapter XV. 

 Darwin was led to examine the subject very carefully inconsequence 

 of Wallace's contention that the variations which led to the special 

 male characters in birds tended at first to be transmitted equally to 

 both sexes, but that the female was prevented from acquiring the 

 conspicuous characters of the male through natural selection, because 

 of the danger she would thus have incurred during incubation. Darwin 

 states that he knew of no facts rendering it probable that a character 

 could be limited to one sex by selection when it was not originally 

 sexually limited in transmission. 



I am not aware that since 1885 any new facts have been pro- 

 duced which would diminish the validity of Darwin's conclusion. We 

 must assume therefore that the variations which give rise to unisexual 

 characters are from their first appearance unisexual in their occurrence 

 and transmission. If we deny the inheritance of acquired characters 

 we can only assume this without any explanation. We can only ob- 

 serve with Darwin that variations occurring late in life are more 

 likely to be unisexually inherited than others, but we can give no 

 reason why changes should occur in the determinants within the germ 

 which produce characters late in life limited in inheritance to one sex. 

 We can only say that they do occur, like other variations equally 

 inherited by both sexes, and that when they occur they may be preser- 

 ved and accumulated by selection. 



Weisrnann in his treatise on the ,,Keimp!asrna" has considered 

 in detail the mechanism of unisexual heredity, but he has not satis- 

 factorily explained the first origin of unisexual variations. He refers 

 to the fact that a character not appearing in the female may yet be 

 transmitted through the female from grandfather to grandson. The ovum 

 from which the female was developed therefore contains the deter- 

 minants of, i. e. the living particles which determine, the male cha- 



