12 INHERITANCE IN RABBITS 



in comparing one set of observations with another. Records of this sort, 

 more or less complete, were made for 70 different litters of rabbits, 

 containing 341 individuals. 



An inspection of figs, i to 3 shows that the growth-curve Jfor ear-length l 

 from 2 weeks after birth is of the same general form in the case of both 

 long-eared and short-eared rabbits. It is a curve convex above, indicating 

 a steadily diminishing daily increment in ear-length. 



GROWTH-RATE OF LOP-EARED AND OF SHORT-EARED RABBITS IN SIZE 

 AND IN EAR-LENGTH. 



The theoretical growth-curve of an organism in weight (Houssay, 107; 

 Robertson, :o8) is at first concave upward, but later becomes convex. 

 When the curve is concave upward the daily growth increment is increas- 

 ing. But when the growth-curve becomes convex upward, it is evident 

 that the growth increment is decreasing. Therefore the period of greatest 

 daily growth occurs when the growth-curve is changing from a concave 

 to a convex one. In rabbits this occurs at an age of from 6 to 8 weeks 

 after birth (see figs, i to 3). According to Robertson (:o8) the period of 

 maximum growth corresponds with the middle point of a growth-cycle 

 which in character resembles an autocatalytic monomolecular chemical 

 reaction. In the rabbit this growth-cycle probably has its beginning at 

 some time prior to birth and ends before puberty is attained. 



It is possible that this same form of curve would be observed in respect 

 to ear-length also, if the measurements began at a period sufficiently early. 

 Growth of the ears is completed before increase in body-weight ceases, 

 and it is possible that the growth-curve for ear-length has already changed 

 from a concave to a convex form at the age of 2 weeks, when our measure- 

 ments begin. But it is, on the other hand, possible that the growth-curve 

 for ear-length would not show a convex form upward even if completed 

 for the period prior to 2 weeks of age; for ear-length is a linear dimen- 

 sion, whereas body-weight depends on volume, i. e. y size in three dimen- 

 sions, and a doubling of any linear dimension should be attended by an 

 eight-fold increase of volume. 



A comparison of fig. i with fig. 2 shows a considerable difference between 

 ordinary short-eared (fig. i) and lop-eared (fig. 2) rabbits as regards size, 

 at corresponding ages; the difference is even more striking in regard to 

 ear-length. Crosses between the two varieties produce rabbits inter- 

 mediate in character as regards both weight and ear-length. But before 

 considering further the character of the cross-breds, it will be well to inquire 

 how each variety breeds by itself. 



1 The measurements for ear- width and "spread" are closely correlated with those for ear-length. 

 For the sake of simplicity we shall deal with the statistics for ear-length only. 



