THE GENERAL SIGNIFICANCE OF AMPHIMIXIS 



207 



area of exemption) and above (the upper limit) can occur, but do 

 not survive and leave descendants, and we can therefore easily 

 understand why characters, of whicli different degrees arise with 

 equal ease from the constitution of the species, must gradually 

 develop a symmetrical curve of frequency because of the constant 

 crossing. Obviously those individuals which stand just upon the 

 borders of admissible variation will, other conditions being equal, 

 leave behind them fewer descendants than those which approximate 

 to the middle of the area of exemption ; for as the characters con- 

 cerned can vary in the offspring in both directions, there will always 

 be at the lower end some of the descendants of a pair which will fall 

 below the limits of exemption, and at the upper end some which will 



Fig. I20. A, symmetrical, and B, asymmetrical curve of frequency ; after 

 Amnion. U, minimal, 0. maximal limit of individual variation. U-0, the 

 'variation-playground.' M, the mean of variation. II, the greatest frequency 

 or mode of variation. 



rise above it. This will happen even when pairing takes place 

 between parents at the middle or at the other end of the abscissa, 

 for there are always cases of the preponderance of one parent in 

 heredity. A higher percentage of the descendants of individuals 

 on the borderline will therefore be eliminated, and their frequency 

 TYiust tlierefore he less. Even if at the beginning of the series of 

 observations a condition obtained in which all the ordinates 

 of the area of exemption were equally high, those nearest the 

 boundaries would of necessity very soon become lower, and this in 

 proportion to their distance from the boundary, and the frequency- 

 curve, which at first would be a straight line (according to our 

 assumption, which of course does not tally with natural conditions). 



