62 PROCEEDINGS OF THE AMERICAN ACADFMY. 



tlio direction of Professor W. E. Castle, to whose guidance and inspiration 

 is due wliatever merit the present contribution may possess. The 

 writer's earlier studies were made on the heredity of complete albinism 

 in its relation to the character pigmentation. It shortly became evident 

 tliat Mendel's principles of heredity, then being brought to notice by 

 certain botanists, were applicable to the case, and this view has since 

 been fully confirmed by more extended observ^ations. The principles of 

 segregation and resolution of characters among hybrids seem funda- 

 mental, and an attempt has been made in this investigation to throw 

 further light on the mechanism of these processes. The writer is aware 

 that the principles enunciated by Mendel, though far-reaching, must not 

 be assumed necessarily to explain all hereditary phenomena, yet they 

 appear to afford the key to much that has previously seemed enigmatical. 

 During the four years since the independent rediscovery of the Men- 

 delian laws in 1900, by the botanists de Vries (: 00, : 00") and Correns 

 (: 00), a considerable literature has sprung up on the subject. The older 

 experiments have been re-examined in the light of the new theory, and 

 other experiments have been instituted to elucidate still further the 

 phenomena of hybridization. At least three recent investigators have 

 experimented with mice, and have published, in part, some of their 

 results. Hence certain of the observations detailed in the present paper 

 are not altogether new, but nevertheless are of value as being corrobora- 

 tive of results independently obtained. The fact that there are several 

 workers in the same field is of further advantage in that an opportunity 

 is offered for the comparison and coordination of results. In the follow- 

 ing pages an attempt is made to explain the phenomena observed, ac- 

 cording to the principles enunciated by Mendel; yet it is not urged that 

 other explanations are impossible. It is simply a question of what 

 hypothesis best explains the facts. 



II. Varieties of Mice and Rats. 



The house mouse (Mas rnusculus), as familiarly known in our dwell- 

 ings and outbuildings, is probably of Eurasian origin, but has been 

 carried by man into all parts of the civilized world. Its color is usually 

 spoken of as gray, the " agouti " of fanciers, and though subject to some 

 slight variation with age and habitat, is nevertheless of a fairly constant 

 type. The pelage of a fully grown specimen taken at Cambridge, Mass., 

 consists dorsally of two sorts of hairs ; the one has a very fine black tip 

 with a suba|)ical I'ingof a light ochraceous color occupying about one-fifth 

 the length of the hair, while the basal portion is dark plumbeous. Seat- 



