EXPERIMENTAL. 29 



Ample sources have been found for the origin of somatic or onto- 

 genetic fluctuations. It will be shown now that there seems to lie 

 behind these superficial changes, to which all the animals are equally 

 subject, the deeper force of heredity determining the size attained by 

 an animal, which produces (despite all possible vitiating influences) the 

 constant difference that has been found in the variability of the two 

 generations. 



MATERIAL. 



In the crosses to be described twelve rabbits were used as original 

 parents. Two of these were males purchased from fanciers. There 

 seems but little doubt that these came from fairly pure stock. One, 

 a Himalayan, is from a race of small animals. As the Himalayan color 

 pattern (white, with dark-colored ears, nose, feet and tail) behaves as 

 a recessive to all other rabbit colors except that of the snow-white 

 albino, there would seem to the fancier to be little advantage in out- 

 crossing. The other male is a black-and-tan, a fairly recent variety of 

 medium size, described by Castle (1909). Little can be said as to the 

 purity of this animal. Since fanciers have definite standards of size 

 for each variety there is strong probability that no wide crossing 

 occurred in the pedigree of this animal. The remaining animals, ten 

 females, were bred in this laboratory in connection with the investi- 

 gations on the inheritance of ear length and color. They came from 

 crosses of more or less diversity in size, although in no case was there 

 a cross between animals with extreme weights, as is shown in tables Za-d. 



CROSSES. 



The small Himalayan male (d"2319) was mated with each of the 

 females. The female offspring (Fi) were bred back to the Himalayan, 

 forming a back-cross generation. This was done on the theory that 

 if any Mendelian phenomena were to take place, a greater number 

 of segregates would have a chance to appear; for, on the basis of a 

 one-character difference between the parents, the second generation 

 obtained by breeding the Fi hybrids together would give the recessive 

 once in four, whereas in a cross between the Fi hybrids and the recessive 

 parent the two types would appear in equal numbers. Similarly, in 

 crosses involving more characters, the back cross to the recessive parent 

 would give a larger proportion of the recessive forms. In three cases 

 Fi males were crossed back to the large female parent. One cross 

 9 1493 X cf 2379, was used as a check; the weights of these animals may 

 be given as 3,000 grams and 2,500 grams. 



