732 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Influence of light of different colors 774 



(a) Influence on fertility 774 



(b) Influence on phototropic responses of larvae 774 



Age at which sexual maturity is attained 775 



Is a male capable of copulating more than once 1 775 



Studies by W. M. Barrows of variation in number of teeth on male sex-comb, 



and of variation in size 776 



Summary 784 



Bibliography 786 



Introduction. 



Physiological characters are inherited no less than morphological 

 ones. In each case there doubtless is in the germ a structural basis on 

 which the development of the peculiarity in question rests. In no in- 

 stance as yet have we been able to identify beyond question the physical 

 basis of any particular character, but a first step in that direction has 

 been taken in the discovery of specific morphogenic substances in the 

 animal egg and of specific differences among the chromosomes of the 

 germinal nuclei in both sexes. While cytologists attack the problem of 

 heredity from the side of the structure of the germ-cells, it is important 

 that their labors be supplemented by a study of the heritable characters 

 themselves, so that the mutual relations of characters and the modifica- 

 tions which they undergo from generation to generation may be better 

 understood. 



Among physiological characters which beyond question are heritable 

 may be mentioned fertility, i. e. the capacity for reproduction. This 

 varies among individuals and among races, as every experienced breeder 

 knows, but the conditions upon which it depends are somewhat uncertain. 

 In some cases external conditions are supposed to induce sterility, as, for 

 example, abundant nutrition and lack of exercise, resulting in excessive 

 vegetative growth without reproductive activity. In other cases inbreed- 

 ing is assigned as a cause of sterility, or sterility may occur spontane- 

 ously without any assignable cause. The relation of inbreeding to 

 sterility has been studied experimentally in mammals by Crampe ('83), 

 Bos ('94), and Guaita ('98) ; and in birds by Fabre-Domengue ('98). 

 They all find the relation to be a causal one, continuous inbreeding, as 

 of brothers and sisters, resulting in decreased fertility, attended more 

 or less commonly by lack of vigor, diminution in size, partial or com- 

 plete sterility, and pathological malformations. It was our expectation 

 that similar effects would be observed in the fly, Drosophila, when 



