H. S. REED 



559 



It will be seen that the observed and calculated values of x agree 

 very well, especially when one bears in mind the extent of variability 

 in biological material. In the latter part of the period the observed 

 values are consistently higher than the calculated. These weight 

 differences are plainly due to the increase in weight during pregnancy. 

 The first parturition occurred at 30 months. As it is, however, the 

 root-mean-square deviation between observed and calculated weights 

 is only 20 pounds. 



135 



110 



en 

 E 85 



60 



20 30 



Time in months 



40 



48 



Fig. 6. Growth rate of Jersey heifers represented by height at withers. The 

 curve shows the values obtained from the equation x = 128 (l —e~^'^^ ^' "^ ^') ; points 

 within circles represent observed heights at various intervals. 



Inquiry may next be directed to the rate at which these animals 

 increased in size and to the value of different kinds of measurements. 

 Eckles and Swett found that the most reliable size measurement was 

 the height of the animal at the withers. The mean height at birth was 

 66.1 cm.; at 4 years it was 125.6 cm. It thus appears that at birth 

 the calf is a little more than half as tall as the mature cow. When we 

 come to study the growth rate it is evident that we cannot use the 

 postnatal Hfe period as the complete growth cycle, because the animal 

 has attained over half its height before birth. Let us assume that 

 height growth begins near the beginning of the gestation period, then 



