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THE AMERICAN NATURALIST [Vol. XLI 



Momentum in Variation. — It is a little late to criticize an article 

 that appeared in November, 1905, nevertheless I should like to say a 

 word or two in regard to the paper by Mr. F. B. Loomis entitled 

 "Momentum in Variation." The conclusion is reached that a varia- 

 tion started along any line tends to carry that line of development to 

 its ultimate, being driven by momentum. If the feature is detrimen- 

 tal, the group dies out. If, however, it is merely a minor feature, it 

 makes a handicap. 



No one doubts that in the course of evolution, specialization goes, 

 so far as to carry a given species or group out of existence, but that 

 this is of such widespread occurrence as Mr. Loomis implies, is open 

 to doubt. Certainly the statements which he adduces to support 

 this theory, are in many cases erroneous and in other instances open 

 to quite other deductions than are placed upon them. 



The few comments here given, are not at all in the line of captious- 

 criticisms, but are merely intended as a protest against any such 

 short-cut to a solution of important problems as that taken in the 

 paper in question. 



If we begin with the Saber-toothed Tigers, which are cited as- 

 examples of extinction due to overdevelopment, we may go back 

 about twenty years to the time when Professor Cope reached a similar 

 conclusion, saying in the course of some discussion that Smilodon 

 undoubtedly became extinct because it could not obtain food, where- 

 upon someone present arose and said: "Mr. Cope, what did the 

 Smilodon feed on"? In connection with this Dr. Matthew has 

 recently brought forward some facts tending to show that the long- 

 tusks of Smilodon were of service in cutting through the long hair and 

 thick hide of some of the contemporary ground sloths. Certainly if 

 the tusks of Smilodon caused its extinction, why does not the Walrus- 

 die out for a similar cause ? The Mammoth with its extreme develop- 

 ment of tusks is also cited to illustrate the principle of momentum in 

 variati6n, leading to extermination, but the great Gangetic elephant 

 which shows the most enormous development of tusks, became extinct 

 long ago, while other members of the race whose tusks were far more 

 recurved lived on. Nor did those mastodons in which the tusks were 

 greatly curved, come to an untimely end one whit sooner than their 

 contemporaries with fairly straight tusks. The African Elephant,, 

 which is much the most primitive in structure of existing species, and 

 more nearly resembles E. ganesa in tusk development, is the species- 

 that has thrived best. Moreover, the African elephant is the one in 

 which tusks are present in both sexes while a large proportion of the 



