280 



THE FOUR INSEPARABLE FACTORS OF EVOLUTION. 



the American Society of Zoologists at the New Haven meeting and published a 

 few weeks later under the title The Four Inseparable Factors of Evolution, Theory 



)f their Distinct and Combined A dim in the Transformation of the Titanothere 

 Extinct Family of Hoofed Animals in the Order Perissodactyla. 



s,an 



Reasons for the Historical Method of Analysis. 



The reason perhaps that this law is connected with the study of an extinct 

 group of animals is that the hypothesis of the simultaneous operation of several 

 factors on different groups of characters could only suggest itself to a palae- 

 ontologist working upon a very complex organism, in which an almost countless 

 number of characters are simultaneously evolving. 



A gross representation of what is observed as to " characters' 1 in the study of 

 a number of different organisms through successive phyletic stages is seen in the 

 following scheme. * 



Progressive Steps, g 



3 



Progressive Steps 





A 12 3 4 + 



B 

 3 2 1 C 



1 2 



D 1 2 3 



4 3 2 1 E 

 5 4 3 2 1 F 



G 



1 2 



H 12 3 4 5 



1 / 

 3 2 1 J 



K 1 2 3 



L 



1 M 



12 3 4 



This 



presentation is g 



because (1) the progressive and retrogressive 



steps instead of being sharply denned, as indicated by numerals 1, 2, 3, 4, and i 

 , are delicately intergraded; (2) the number of characters is very much larg 



on 



than indicated above. It is estimated that there are at least 560 independent 

 " characters' 1 evolving simultaneously in the grinding teeth alone. 



The palaeontologist is in a peculiarly favorable position to observe both the 

 origins of new characters and the transformations of existing characters as 

 visible modes of evolution: the palaeontologist enjoys the unique advantage denied 

 to all other zoologists of being present before the birth, at the birth, during the 

 progress, into the rise, decline and death of new characters and organs. 



The genesis of single characters is apparently a simple, concrete problem. 

 When and how does a new cusp arise on a grinding tooth? When and how does 

 a new horn arise on a skull? When and how does a broad skull change into a 

 long skull, and vice versa? 



In seeking, however, to explain the conditions or causes which give rise to 

 these things it is necessary for the palaeontologist to remind himself constantly of 

 the following important distinction : 



i 



