1 80 Heredity. 



opposite sex, ready to be evolved under peculiar circumstances. 

 ' We can thus understand how, for instance, it is possible for a good 

 milking cow to transmit her good qualities through her male off- 

 spring to future generations, for we may confidently believe that 

 these qualities are present, though latent, in the males of each 

 generation. So it is with the game-cock, who can transmit his 

 superiority in courage and vigour through his female to his male 

 offspring.' 1 



As Darwin remarks, these facts oblige us to admit that certain 

 characters, aptitudes, and instincts may remain in the latent state 

 in an individual, and even in a series of individuals, while yet we 

 are unable to find any trace of their presence ; and on this hypo- 

 thesis the transmission of a characteristic from grandfather to grand- 

 child, with the apparent omission in the intermediate parent of the 

 opposite sex, becomes very plain. 



What has now been said respecting latent characteristics applies 

 to a form of heredity of which we have not yet treated specifically, 

 heredity occurring at corresponding periods. This, it appears to 

 us, may be explained on the hypothesis of latent characteristics 

 contained in the individual in the germ state, and which come to 

 light only under definite conditions, and at some particular point 

 of his development, and this particular moment corresponding 

 with a similar moment in the progenitors. Hereditary diseases 

 are a good instance of heredity at corresponding periods. Thus, 

 chorea, which usually makes its appearance in childhood, con- 

 sumption in middle age, gout in old age, are naturally hereditaiy 

 in the same periods. 



Blindness furnishes still more striking instances. In one family 

 it was hereditary for three generations, and thirty-seven children 

 and grandchildren became blind between their seventeenth and 

 eighteenth year. In another instance, a father and his four 

 children were all attacked with blindness at the age of twenty-one. 

 It is the same with deafness. Two brothers, their father, their 

 paternal grandfather, all became deaf at the age of forty. 2 Esquirol 



1 Variation, etc., ii. 



a Dr. Sedgwick, British and Foreign Medical and Chirurgical Review, 1861, 

 p. 485. See also Lucas ii. 739, and Darwin Variation, etc., ii. 80 



