No. 610] GENETICS VERSUS PALEONTOLOGY 629 



forms were found living at the same time in a restricted 

 area one might perhaps have suspected that these contem- 

 poraneous intermediate forms were parallel offshoots of 

 a normal parent stock rather than linear descendants one 

 of another. 



It may well be true that, until it can be shown that evo- 

 lution '*in the open" is continuous and not discontinuous, 

 all ''laws" and ''principles" which merely assume such 

 continuity are open to question. But there is considerable 

 evidence for the conclusion that many races of mammals 

 have evolved either quite continuously or by small suc- 

 cessive gradations. It is true that in some cases appar- 

 ently new and distinct forms also appear in successive 

 horizons, but these new forms may be immigrants from 

 other distribution centers^— the little-modified descend- 

 ants of indigenous races being often found side by side 

 with their more progressive immigrant relatives. 



The great collections of American Eocene and later 

 mammals which have been brought together by the sys- 

 tematic explorations of the American Museum of Natural 

 History are all exactly recorded as to level, so that ex- 

 cept in a few instances there can be no doubt whatever as 

 to the chronological sequence of the specimens. These 

 collections, numbering many thousands of specimens, are 

 being minutely studied by several investigators, who are 

 not trying to prove any theory of evolution, but are re- 

 cording and identifying specimens and analyzing their 

 observations, with such accuracy and judgment as they 

 may have gained from twenty years of experience in this 

 work. 



The results of these studies, as bearing on the question 

 of continuity vs. discontinuity in evolution, are too ex- 

 tensive and complex to be summarized here, but a few 

 examples may serve to illustrate the kind of evidence 

 available and the conclusions which have been drawn in 

 typical instances. 



Very often as we pass from lower to higher strata of 



4 Matthew, W. D., "The Continuity of Development," The Populur 

 Science Monthly, Xov., 1910, pp. 473-478. 



