DOES ADVANTAGE IN UNIFORMITY EXPLAIN DIVERGENCE? 261 



right-handedness, there is a distinct advantage in uniformity, and that consideration 

 alone may perhaps suffice to explain Mr. Gulick's difficulty. Among plants it 

 may seem less obvious, but where seedlings are crowded, uniformity may save 

 space, just as a number of objects of the same shape can usually be packed into 

 less space than those of diverse shapes. More plants can grow in a window-box 

 when all bend to the light than would be possible if half of them bent one way 

 and half another. 



There also occurs to me a theoretical consideration, perhaps of doubtful value. 

 When a germ has diverse potentialities, so that it is left to germinal or environ- 

 mental selection to decide which course it shall take in development, there must 

 apparently be a certain waste of germinal energy. Any disadvantage thus arising 

 is ordinarily much more than counterbalanced by the gain due to the adapta- 

 bility of the organism, or in social species to the power of specialization of the in- 

 dividual for social purposes. But it may be that when no such advantage is 

 found, there exists a small disadvantage in deviations, potential or axial, from a 

 common standard. 



What we really need, in discussing these matters, is the observation of actual 

 facts. The facts above related as to Campeloma are worth more than any amount 



of theoretical considerations. 



T. D. A. COCKERELL. 



MESILLA, NEW MEXICO, U. S. A., April 21, 1897. 



6. Reply to Letter of T. D. A. Cockerell. 



The advantage in uniformity is very manifest in certain cases; and, on pages 

 68-70 of this volume, I refer to conditions in which it is more manifest than in 

 the cases here suggested by Mr. Cockerell; but the advantage of uniformity 

 does not "explain the difficulty" I have raised. For how can the advantage 

 of uniformity explain the introduction of permanent diversity through the sur- 

 vival of a variation that breaks down the former uniformity, and establishes two 

 forms where there was a single form ? 



The disadvantage in deviation from a common standard, if it can be shown to 

 be a fact, is perhaps akin to the fact that variations most widely diverging from 

 the average form are usually less fertile. But how can the advantage of a com- 

 mon standard cause the dividing of a species according to two different standards 

 as in the case of some snails? For any one snail of a dextral group there may be a 

 disadvantage in being of a sinistral form; but does that throw any light on why a 

 species should, under one environment, divide itself into two groups, one being 

 dextral and the other sinistral? and does it show that the process is due to nat- 

 ural selection? The best explanation I can suggest is given on pages 68-70. 



J. T. GUUCK. 



