184 E. C. MACDOWELL 



EXPERIMENTAL 



In 1908 Castle started a second experiment that should test 

 this hypothesis by obtaining larger numbers of animals so that 

 the variabilities of the first and second generation could be com- 

 pared. As the size of the whole animal was to be considered, 

 crosses were made between a small male rabbit from a small 

 race, the Himalayan, and a series of large females that had been 

 used in the ear length crosses. The pedigrees of the females are 

 known and for several generations there were only slight differ- 

 ences in the weights of the pairs of ancestors. In 1910 the ex- 

 periment was put into the hands of the writer. By that time 

 the original crosses had been made and weights for growth curves 

 had been recorded for the animals raised up to that time. I 

 wish to acknowledge here a keen appreciation of my indebtedness 

 to Professor Castle for the privilege of completing this work 

 which he had planned and already started, and to express gratitude 

 for the assistance and advice that he has contributed towards 

 the completion of this work. 



In most cases the second generation consisted of a back cross 

 of the first generation females to their male parent. A few back 

 crosses of first generation males to their female parent were 

 made and a few crosses between first generation males and fe- 

 males. Measurements were made of the skull and long bones of 

 the rabbits after the bones had attained full size. Fifteen months 

 was set as the age for killing a rabbit, as it was found that the 

 bones were fully grown at that age. Sixteen measurements for 

 each set of bones were recorded. 



a. Coefficients of size 



It was found that there was enough lack of correlation between 

 the various measurements to give different results when different 

 characters were considered, so it became needful to obtain a 

 number for each animal in which the various measurements would 

 be equally represented to express the size of each animal. As 

 the measurements varied from 2 to 10 cm. no absolute average 

 could be used; for a small deviation in a short measurement 



