8 



GENETIC STUDIES OF RABBITS AND RATS. 



rabbit raised. From these curves, readings have been taken giving 

 the approximate weight of the individual at the ages of 30, 40, 60 

 days, and so on, at 30-day intervals, to the age 360 days. Combining 

 the weight records for each group of individuals, an average growth- 

 curve is obtained for the group, as shown in figure 3. 



In this figure it is indicated that Polish rabbits are small at 30 

 days of age, but grow with fair rapidity until about 180 days old. 

 Then the growth-rate declines and growth is practically ended at the 

 age 210 days. In many cases the weight actually decreases again 

 after attaining a maximum at sexual maturity, or shortly thereafter, 

 as Punnett (1918) has observed. But such decline is not invariable, 

 and it may result from a variety of causes, such as less nutritious 



35 



30 



25 



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 pzo 



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15 



10 



FiQ. 3. — Growth-curves of the 

 three races of rabbits, Polish, 

 Himalayan, and Flemish, 

 and of the groups of Fi hy- 

 brids obtained by crossing 

 the three pure races. The 

 average adult weight of each 

 group of Fj hybrids is shown 

 for comparison with Fi, be- 

 low which it falls in every 

 case. 



3040 60 90 



tzo ISO leo 210 



AGE, IN DAYS 



240 270 300 330 360 



food, a cold, or lactation (in females). When normal conditions are 

 restored, the weight usually rises again and subsequent accumulations 

 of fat usually carry the final weight above the puberty maximum. 

 In plotting the growth-curves the maximum weight attained in the 

 first year is considered as persisting thereafter. This will account 

 for the fact that the plotted average curves do not show any decline, 

 even though the weight-curves of individuals would in many cases 

 do so. The rabbits that show increases in weight, due largely to 

 fattening subsequent to puberty (180 to 210 days), will explain the 

 fact that the weight-curve continues to rise until the end of the period 

 plotted, 360 days. 



In the case of Himalayan and Flemish rabbits, the weight at the 

 age 30 days is greater than that of Polish rabbits. The growth-rate 

 is also greater, so that the growth-curves continue to diverge more 

 and more, one from another. The slowing up of growth at puberty 



