216 THE AMERICAN NATURALIST [Vol. LII 



manently according to a definite standard since it is im- 

 possible to secure a constant environment. A fairly uni- 

 form environment in the sense that all birds are exposed 

 to the same external conditions at any one moment is 

 fairly easily secured; but since the external conditions, 

 particularly weather conditions, are very variable and 

 follow no definite course, and since a bird's vigor is a 

 resultant of its own inherent strength of resistance 

 against the environment, it is clear that the objective 

 vitality observed in each member of a flock may be un- 

 equally affected by the surroundings. 



The evidence available on the relation of vitality to 

 fecundity thus far points in two more or less opposite 

 directions. Many birds of low vitality have made not 

 only excellent but even high records. On the other hand, 

 birds of strong vitality may make low records. At the 

 same time there is a point at which the vitality becomes 

 too low for good egg production. In the fall of 1913, 

 thirty-eight birds graded early in the fall, before laying 

 commenced, as "poor" in respect to vigor, were put in 

 the laying houses. They had an average record for the 

 winter period of only 20 eggs against an average of 38 

 for the entire flock, including the poor birds. Low vital- 

 ity evidently depressed egg production in this instance; 

 mainly, through retardation of the time of commence- 

 ment of laying rather than by slow production. The in- 

 fluence of lack of vigor on winter egg production is shown 

 in Fig. 9, where the curve of winter egg production for 

 the entire flock is represented by the continuous line and 

 that for the "poor" birds by the dotted line. 



Occasionally, birds of low vitality may make excellent 

 egg records. In one family in particular, the birds were 

 of distinctly mediocre quality, as evidenced by their 

 weight, activity, hatching quality of eggs and viability of 

 chicks and yet they were able to make high records, the 

 average for the family of seven individuals being 63.3— 

 ranging from 33 to 81— for the winter period, with a 

 yearly average of 192.4 for the five birds that survived 

 throughout the year, and with a range of 154 to 210. 



