442 



THE AMERICAN NATURALIST 



[Vol. L 



wholly wanting in the flora of the latter region. From these 

 climatic facts it appears that while soil moisture is a condition 

 sine qua non of the presence of the cacti, the range of the actual 

 amount of soil moisture must he very great indeed, so, in short, 

 it results that the temperature is the factor in direct control, thus 

 a very important limiting factor. 



Should we sum up, therefore, the factors thus far mentioned 

 as being important among those which determine the distribu- 

 tion of the cacti, we find, in the first place, that the shallowly 

 placed root-system subjects the roots to the greatest possible ex- 

 tremes in soil temperatures, including those that are high, and, at 

 the same time, makes it possible for the plants to advantage from 

 the minimum effective rainfall. Further, an effective growth rate 

 of the roots takes place only at relatively high soil temperatures. 

 .And, finally, a certain but highly variable amount of moisture 

 must be present in the soil. Since the crux of the matter, how- 

 ever, appears to be the fact that the root-system of the cacti are 

 essentially superficial, there is the additional factor, or factors, 

 which bring about this circumstance. These are at present un- 

 proved, but the results of experimental studies, not published, 

 indicate that among them must be included the response to the 

 oxygen supply of the soil. 



W. A. Cannon 



THE INHERITANCE OF CONGENITAL CATARACT 



In the February number of the American Naturalist there 

 is an article from the Bussey Institution by Jones and Mason 1 

 in which an attempt is made to show that congenital cataract he- 

 haves in heredity as a simple Mendelian recessive. The authors 

 from a study of family histories published by Harman in the 

 "Treasury of Human Inheritance" come to conclusions at vari- 

 ance with those of Bateson and Davenport, which authors they 

 are perhaps unjustly disposed to criticize. The paper is well 

 written and embodies a considerable mass of data, so that the 

 reader not familiar with this particular problem might easily be 

 led to think that the older investigators had really made a mis- 

 take in interpretation. The evidence, however, does not seem to 



i Jones, D. F., and Mason, S. L., "Inheritance of Congenital Cataract," 

 The American Naturalist, Vol. L, No. 590, pp. 119-126, February, 1916. 



