418 Grrowth and Development of Breeds and Crosses ofPif/s 



Table XVIII. Changes in the proportional development of Berkshire 

 ■pigs from Period I (1903-6) to Period II (1907-13). 

 Percentage of live weight 

 3 munths 5 months 7 months 9 months 1 1 months 



Individual variation. All the figures given in this paper are averages 

 of a number of pigs and there is a large amount of individual variation 

 in weight apart from those due to causes which are known. Such things 

 as strain, state of fatness, etc., influence the hve weight and all these 

 come under the heading of individual variation. One of the most im- 

 portant causes of variation is the amount of food contained in the 

 aHmentary canal ; while it is not so important as in cattle and sheep where 

 the contained food may be as much as 14 per cent, of the live weight yet 

 it is a large factor. Table B shows that it may vary from 2 to 11 per cent, 

 of the live weight ; while not all of this can be eliminated yet it is sufficient 

 to cause considerable variation. 



As a measure of this variation in the various breeds and at different 

 ages the standard deviation has been calculated and is shown in Table XIX; 

 the coefficient of variability has been used as the measure of variation. 

 The Large Black breed shows greater variation than the others and this 

 is probably in part due to the increases in weight which have been made 

 in this breed in the last few years (see Selection above). It is notable that 

 the first crosses show rather less variation than the pure breeds. 



On the average of all breeds the coefficient of variability increases 

 with age up to 7 months and then decreases again, variability being 

 greatest when the rate of growth is at its maximum. This will be seen if 

 the coefficients of variability at the base of Table XIX are compared 

 with the rates of growth at the different ages given in Fig. 2. There is, 

 however, more variation at the younger ages than at the older ones, as 

 might be expected if improvements both by feeding and breeding were 

 being made in the rate of early maturity. That variability is greatest 

 when the rate of increase is at its maximum was first pointed out by 

 King (67) for rats. Robertson's (68) results also show it to be true for mice 



