PART 1 1. -EXPERIMENTAL. 

 INTRODUCTION. 



In the following report of an investigation of the inheritance of the 

 weight and of the size of various skeletal characters of rabbits, facts 

 are presented that are believed to indicate an increase in variability 

 as a result of size crosses. The work here described was started that 

 further knowledge of the inheritance of the size of rabbits might be 

 obtained, and, in the light of the evidence presented by Nilsson-Ehle, 

 to test the hypothesis that differences in size may be due to the various 

 groupings of similar independent Mendelian factors. 



The original crosses of the large and small rabbits and the plan for 

 breeding were made by Professor W. E. Castle in November 1908, who 

 kept the records of their progeny until January 1910, when the investi- 

 gation was put into my hands. With the exception of the recording of 

 the weights during seven weeks in the summer of 1910, which was very 

 generously done by Professor Castle, I have made all observations since 

 1909; I wish to acknowledge here a keen appreciation of my indebted- 

 ness to Professor Castle for the privilege of completing this work, which 

 was already planned and started, and to express my deep gratitude for 

 the untold assistance and advice that he has given freely during the 

 course of these experiments. 



ROLE OF ENVIRONMENT. 



In any study of size or weight the control of food, temperature, and 

 general surroundings is very essential. Valuable information on the 

 actual influence of environment is given by a series of curves that have 

 been made from a series of weighings of each animal (see p. 43). In the 

 case of rabbits irregularities are very marked. Guinea pigs raised 

 under identical conditions show much smoother curves. This differ- 

 ence may be due largely to the considerable mass of material constantly 

 in the large caecum of the rabbit, which is directly affected by any 

 unfavorable condition that would cause a loss of appetite. So this 

 caecum would act as a magnifier of the actual fluctuations in the weight 

 of the tissues themselves. 



The rabbits were raised in two adjoining rooms, in which temperature 

 and light conditions were similar. The rooms were heated by steam 

 in winter and in summer were cooler than outdoors. The animals were 

 all fed regularly on the same rations, with a change from fresh grass 

 in summer to hay and mangels in winter. Oats were given them all 



