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. 
60/7 
[oN 
on 
=) 
T 
ic 
Pa) 
roe 
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\ 
| 
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() 
Mean Winter Production 
g 
| 
10 
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Year 709-00 199291 0102 499293 70304 19.4295 05°06 196297707708 r9g299 0910 94 92y 12 9914311314 94 4215 
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- 
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