456 



GENETICS IN RELATION TO AGRICULTURE 



since calving, gave 13 per cent, more milk and 12 per cent, more butter 

 than those scrubs which were brought to the station as mature cows. 

 The numbers are not large, but the test as far as each individual was 

 concerned was fair and extensive, covering for the most part four lactation 

 periods. Since the animals while under test were given the same kind 

 of treatment, we may justly conclude that the unfavorable development 

 of those scrubs which were brought to the station as mature cows had 

 permanently lowered their milk producing capacity. This is by no 

 means a surprising conclusion; on the contrary, it is exactly what would 

 have been expected. The same relations obtain in all other characters 

 in domestic animals; modifiability in its effect may be either permanent 

 or transient. 



Q0101 ' Oi: 2 -02--03 >03!04 -04105 ^^ WOT* 07 ' 08 '08-'09 'O 9 '' 10 'lOUl' 11 ' 12 '12J13 13!l4 'li'-lt 



FIG. 185. A graphic representation of the results of breeding for high winter egg 

 production. The period from 1899 to 1907, that of mass selection: 1908 to 1915 of geno- 

 typic selection; the dotted line for low production, the broken line for high production. 

 (After Pearl.) 



Modifiability and Breeding Value. Often modifiability is of value 

 to the herdsman in selecting his breeding stock, because it enables him 

 to magnify differences between different individuals, and, therefore, to 

 select his animals more accurately. For if modifiability is proportional 

 to genetic variability, the increased production under forced conditions 

 should be merely a somatic expression of the genetic potentialities of the 

 individual. The trouble, however, is that variability of this kind is 

 very often erratic in character, so much so that forced production, or 

 even performance under more normal conditions, is often a very unsafe 

 guide, except when reinforced by a knowledge of family history. 



A most striking case is reported from the Maine Station where breed- 



