6 BIOLOGICAL LECTURES. 



trary, a species is unstable and has a general tendency to 

 vary, the base will be long. 



The 868 American eggs arrange themselves in respect to 

 lengths as represented by the broken line on Diagram I. The 

 base of this curve is long. Its summit coincides with the ordinate 

 of 21 mm. Its interest, of course, lies chiefly in the relation- 

 ship it bears to the curve of British eggs. 



The latter curve is represented by an unbroken line. Its 

 base extends from the ordinate of 18.5 mm. to the ordinate of 

 25 mm., and its point of greatest altitude is upon the ordinate 

 of 22 mm. 



A moment's examination of these curves reveals not only 

 the fact that the American eggs are more variable, i.e., the 

 base of the dotted curve is broader, but it also yields data appro- 

 priate to the third and fourth propositions ; for it will be 

 observed that the American eggs have undergone a striking 

 reduction in their average length, that is, they show a departure 

 from a previously maintained higher standard, viz., 22 mm. in 

 length, and they are also tending to gather about a new point 

 of equilibrium, viz., 21 mm. in length. 



Without commenting upon these observations, which are 

 based upon absolute measurements, let us see if the ratio of the 

 breadth of the egg to the length, that is, the shape of the egg, 

 has also been affected by the withdrawal of natural selection. 



The curves on Diagram II are designed to represent the dis- 

 tribution of eggs according to the ratio of their major and 

 minor diameters. When an egg approaches sphericity, the 

 ratio is higher; when it is elongated, the ratio is lower. The 

 more elongated eggs are arranged at the right of the diagram ; 

 the short, stumpy ones are arranged at the left. Oval and 

 ellipsoidal eggs naturally occupy positions along the middle 

 ordinates. The broken line, as before, represents the distribu- 

 tion of American eggs, the unbroken line, of British. 



On this diagram it will be noted that the American eggs 

 again show a greater amplitude of variation, the base of the 

 dotted curve being nearly one-fifth broader than that of the en- 

 tire curve. It will also be noted that, appropriate to the third 

 proposition, the American eggs have undergone a striking 



