148 ESSAYS OF A BIOLOGIST 



possibility. We know that in various lower animals, 

 such as moths and flies, the balance between the 

 male- and female-determining factors in the chro- 

 mosomes may be altered in certain crosses, and that 

 this altered balance in the constitution is reflected 

 in some cases in a state permanently intermediate 

 between male and female, in others by a reversal of 

 sex at some point during development. For various 

 reasons we should not usually expect reversal in 

 mammals; but if such abnormal balance should ex- 

 ist in the constitution, as it well might, we should 

 expect a gonad secreting an abnormal, intermediate 

 secretion. This we might also expect as the result 

 of certain accidents of embryonic life, as actually 

 happens in the abnormal female cattle known to 

 farmers as free-martins. These animals are always 

 born co-twin to a male, and their abnormality is due 

 to the blood-systems of the embryonic membranes 

 of the twins having fused, so that the secretion of 

 the developing male's gonad acts upon the develop- 

 ing female. 



Further light on abnormally-directed sex-instinct 

 is thrown by recent analysis of abnormal domestic 

 animals by Crew.^^ \^ both goats and swine he fmds 

 that by far the commonest form of sexual abnor- 

 mality is one in which the external appearance, at 

 least in youth, is so nearly female as to raise no 

 question in the mind of the casual observer; about 

 the time of maturity, however, male secondary sex 



12 Crew, '23. 



