A PHYSIOLOGICAL CHARACTER. 



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FIGURE 2. Line 695. 



A. Relative rates of descent of the two strains. The base-line represents an equality of 

 descent, i. e., if both strains produced young of the same generation of descent on the same 

 day the circle would fall on the base-line with a figure "0" placed adjacent. Positions above 

 the base-line (solid circles) indicate that the minus strain in a particular generation was lagging 

 by as many generations as the point falls in units below the base-line and by as many additional 

 days as are indicated by the adjacent numeral. The horizontal scaling indicates the two-month 

 periods of the experiment. 



B. Reproductive indices plotted to show which strain is superior in vigor (and by how much 

 it is superior) in the different two-month periods in experiment. Solid circles indicate that the 

 plus strain is superior by the amount indicated by the distance the solid circle lies above the base- 

 line; conversely, open circles below the base-line indicate the amount of superiority of the minus 

 strain. The horizontal scale indicates, as always, the two-month periods of the experiment. 



C. Reaction-time curves. The plus strain in solid, the minus strain in broken line. The 

 vertical scale indicates seconds of reaction-time, the horizontal scale two-month periods of the 

 experiment. The points in the curves represent mean individual reaction-times by two-month 

 periods. The roman numerals indicate times at which "test series" were conducted, and the 

 plus and minus signs the positions for the test series means for the two strains. 



It seemed desirable to combine these measures so as to write the 

 result in a single quantitative term. This was in part accomplished 

 by dividing the mean number of young in the first brood by the mean 

 age of the mothers at the time the first broods were produced. This 

 gives the average number of young per day of the mother's age and 

 may be called the reproductive index. 1 This reproductive index is 



1 Unfortunately this did not include data for the interval between the first and second 

 broods. This, however, was the least important of the three available measures of vigor, and 

 the data for it was less extensive than for the other two measures. 



