GENETIC STUDIES OF RABBITS AND RATS. 



PART I. 



SIZE-INHERITANCE IN RABBIT CROSSES. 



In the last fifteen years numerous studies have been made of the 

 inheritance of characters which are quantitatively variable or fluc- 

 tuating. As a result of these studies it has become clear that in 

 many cases fluctuation is due to non-genetic causes, to the environ- 

 ment rather than to the constitution of the germ-cells. In such 

 cases selection is without effect in modifying the racial character. 

 This is the accepted explanation of the negative results obtained by 

 Johannsen in selecting beans for increased or decreased size, and of 

 the similar results of Ewing in selecting plant-lice for altered body 

 dimensions, and those of Jennings and others with paramecium. 



But in a majority of cases variation due to genetic causes occurs 

 in association with that due to non-genetic or environmental causes; 

 in fact, it is possible to distinguish between the two only by the 

 results of systematic selection. When the environment is kept 

 constant and a race does not change in response to selection, we 

 assume that no genetic variation is present. But if the race does 

 change in response to selection, we have no alternative but to assume 

 that the variation is genetic in character. Body-size in birds and 

 mammals shows well the simultaneous yet distinct action of genetic 

 and non-genetic agencies. The amount and quality of the food 

 supplied to an animal limits its size, yet if food is supplied in abun- 

 dance and of proper quality, some races of animals attain greater 

 size than others. This is the result of genetic differences. 



The analysis of such genetic differences is difficult. A pioneer 

 attempt was made by Galton (1889) in his study of human stature, 

 the inheritance of which he characterized as blending. This term 

 was adopted by Castle et al. (1909) in describing the inheritance 

 of ear-length and body-size in rabbits. A Mendelian interpretation 

 of size-inheritance was later advocated by Lang (1910), based on 

 the multiple-factor hypothesis of Nilsson-Ehle and this has now 

 received general acceptance. Davenport (1917) has recently applied 

 it to human stature, but has gone a step farther in assuming that 

 the genetic factors which govern size in one part of the body are 

 often not the same as those which govern the size of other parts. 



Punnett and Bailey (1914, 1918) in their studies of size-inheritance 

 in poultry and in rabbits bring forward a different hypothesis. 



