Full text: Problems in eugenics

5┬░Section I.R. Pearl. 
conditions which taken together determine the laying of about such a 
number of eggs as represents the normal reproductive activity of the wild 
Gallus bankiva. It must be remembered that, for reasons which cannot be 
gone into here, under conditions of domestication the activity of this normal 
ovulation factor will mean the production of considerably more eggs than 
under wild conditions. Egg production involves certain definite and rather 
severe metabolic demands, which under wild conditions will not always, or 
even often be met. Further, as has been especially emphasized by Herrick, 
egg laying in wild birds is simply one phase of a cyclical process. If the 
cycle is not disturbed in any way the egg production is simply the minimum 
required for the perpetuation of the race. If, however, the cycle is 
disturbed, as for example, by the eggs being removed from the nest as fast 
as they are laid, a very considerable increase in the total number of eggs 
produced will result. 
It is a fact well known to poultrymen, and one capable of easy 
observation and confirmation, that different breeds and strains of poultry 
differ widely in their laying capacity. In saying this the writer would not 
be understood to affirm that a definite degree of fecundity is a fixed and 
unalterable characteristic by any particular breed. The history of breeds 
shows very clearly that certain breeds now notably poor in laying qualities 
were once particularly good. One of the best examples of this is the 
Polish fowl. But, in spite of this, not only do these breed and strain 
differences in fecundity exist, and probably always have existed, but they 
are inherited. Such inherited differences are independent of feeding or any 
other environmental factors. Thus a strain of Cornish Indian Games with 
which I have worked are poor layers, regardless of how they are fed and 
handled. This is merely a statement of particular fact; it does not imply 
that there may not exist other strains of Cornish Indian Games that are 
good layers. 
Now in individuals which are high layers, and have this characteristic 
in hereditary form, there must be involved some sort of physiological factor 
in addition to the normal ovulation factor already discussed. An analysis 
of extensive statistics has shown that high fecundity represents essentially 
an addition of two definite seasonal, laying cycles to the basic normal 
reproduction cycles. These added periods of productivity are what may be 
called the winter cycle and the summer cycle. The winter cycle is the more 
important of these. It is the best measure of relative fecundity which we 
have and has been used as the chief unit of fecundity in these studies. 
It constitutes a distinct and definite entity in fecundity curves. The 
existence of these added fecundity cycles in high laying birds must depend 
upon some additional physiological factor or mechanism besides that which 
suffices for the normal reproductive egg production. Given the basic 
anatomical and physiological factors the bird only lays a large number of 
eggs if an additional factor is present.

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