8 BIOLOGICAL LECTURES. 



change in shape, as indicated by the ratio of breadth to length; 

 and, appropriate to the fourth proposition, that the American 

 eggs are not indiscriminately distributed, but tend to gather 

 about a mean type. This type is located on or near the ordi- 

 nate of 73/0, and is removed some little distance toward the 

 side of sphericity, and away from the correlative ordinate (70/0) 

 of the British specimens. 



The second curves, then, bring out in a more emphatic way 

 the same general facts that were shown on the first diagram. 



But it is quite evident that the mere ratio of breadth to 

 length is not an adequate index of variation in shape. On this 

 ratio alone, an ^gg that is conical, or pear-shaped, may not 

 appear in any way different from one that is ellipsoidal or 

 lemon-shaped. I have made several attempts to bring out 

 these extreme variations in some practical arithmetical manner, 

 but have felt each time that the eggs varied far more than the 

 numerical results indicated. 



For want of a better method, I finally adopted the following: 



Having placed upon each American ^gg a secret mark, the 

 eggs of both countries were thoroughly mixed together in a 

 single tray. A disinterested person was then requested to 

 select, from the mixture of 1736 eggs, one hundred eggs which 

 appeared to him to present extremes of shape-variation. If 

 eggs from the two countries are equally variable, it is clear that 

 approximately the same number from each would be selected; 

 and, of course, if the American eggs are more variable, more 

 American eggs would be selected. The result of this experi- 

 ment was most striking, and in harmony with the evidence 

 derived from the comparison of lengths and the ratios of 

 breadth to length. Eighty-one of the selected eggs were 

 American, while only nineteen were English; over four times 

 as many of the former as of the latter. 



As before mentioned, the colors of both European and 

 American eggs are subject to variation, arising from modifica- 

 tions of the ground color and from the color and distribution 

 of the spots or blotches. Some are of a somber color, much 

 like the eggs of our common song sparrow; others resemble 

 the eggs of the kingbird; and still others have the delicate 



