78 VARIATION 



are not the same for any two species (functional variation between 

 species). 



Again, no two individuals of the same species can be depended 

 upon to give exactly the same quality of milk, for herd records 

 show that the milk of different cows varies naturally from less 

 than 3 per cent to more than 6 per cent fat l (functional varia- 

 tion between individuals). Nor is this dependent upon the food 

 supply, for all authorities agree that the proportion of fat to 

 other solids is dependent upon the individual and not upon her 

 feed. Moreover, differences nearly as wide as these quoted may 

 be found within the limits of a single herd and therefore under 

 identical conditions as to feed. 



Still again, two individuals of the same breed will produce 

 radically different amounts of milk or fat, whichever is measured, 

 from identical amounts of the same kind of feed. This has been 

 repeatedly and conclusively shown by Professor Eraser of the 

 University of Illinois. 2 Probably no fact in animal physiology is 

 of more far-reaching importance than is this marked instance of 

 functional difference between individuals. 



Three experiments were conducted in the attempt to deter- 

 mine the limits of this difference between cows considered good 

 enough for a place in a commercial herd. In the first 3 Eva 

 produced 48 per cent more milk and 1 1 per cent more butter 

 in ninety-one days than did Janet, and in doing so consumed no 

 more grain and but 7.6 per cent more roughness. These cows 

 were both mature, were fresh on the same day, and neither suf- 

 fered accident during the experiment, yet Eva produced 1057 

 pounds of milk and 12 pounds of fat out of an extra feed of 

 112 pounds of hay and corn stover, a difference greater than 

 any margin of profit the dairyman may hope to realize. 



The second experiment was a comparison between Rose, a 

 native cow nine years old, and Nora, a native cow six years old. 4 



1 The actual range in milk is far greater than these figures. Single milkings 

 have been known to run as low as 1.8 per cent fat, and Jersey cows near the close 

 of lactation often give milk with 9.0 per cent fat. 



2 See Bulletin No. 51 and Bulletin No. 66, Agricultural Experiment Station, 

 University of Illinois, May, 1898. 



8 Ibid. (51) p. 103. 



* Bulletin No. 66, University of Illinois, November, 1901. 



