578 GENETICS IN RELATION TO AGRICULTURE 
that eventually a given result may be produced, his time is limited and 
he, therefore, desires, and rightly, to achieve a given result in the shortest 
possible ‘time. A case in point is that which we have already discussed 
in some detail, modifiability in relation to selection for high egg produc- 
tion in the domestic fowl. Here Pearl found that modifiability was so 
great that simple phenotypic selection of the highest producers for breed- 
ing stock resulted in no improvement whatever in laying capacity. On 
the other hand, the application of a method of breeding which fully 
allowed for this effect of modifiability and which further took into account 
the germinal constitutions of the fowls selected for breeding purposes 
immediately resulted in gratifying improvement. With most characters 
the influence of modifiability is not so great as in fecundity of fowls. 
Often in fact modifiability may actually be utilized to advantage by the 
breeder in determining relative excellence. Thus any system of develop- 
ment which tends to call forth the highest possible expression of the 
capabilities of individuals tends to widen the differences between superior 
and inferior individuals. Both good and poor dairy cows tend to give 
increased milk yield when fed richly, but the increase is often more marked 
in the good cows. On the other hand, with horses in general it is possible 
by training to increase speed, but it is a question whether the increase in 
such a case is more marked in good or poor horses. We may say with 
confidence, however, that here training by developing the full capa- 
bilities of the animal tends to bring its speed up to such a standard 
that when compared with breed records, the superior excellence of the 
individual is definitely established. Modifiability, therefore, is on the 
one hand a factor which tends to decrease the possible effectiveness of the 
method of breeding from the best; on the other hand, if properly utilized 
it is a powerful aid in the accurate selection of those individuals which 
possess the highest inborn capabilities. 
When we come to consider the influence of germinal diversity in 
phenotypic selection, we approach more nearly the problem of the real 
limitations under which the method of phenotypic selection labors. Here 
we may distinguish different ways in which germinal diversity may hinder 
phenotypic selection. 
Phenotygic Selection Does Not Distinguish Between Homozygous and 
Heterozygous Individuals—To the student of Mendelism this diffi- 
culty requires no further comment. It may be pointed out, however, 
that the difficulty increases as the number of factors for which selection 
is being practised increases. As with modifiability, however, this diffi- 
culty tends to be obliterated by long-continued selection, for such 
selection inevitably increases the proportion of homozygous individuals 
within a given phenotype or standard of selection. Roughly it may be 
said that the rate of increase of the proportion of homozygous individuals 
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