GENE MUTATIONS AND EVOLUTION 87 



who has recently published a number of papers presenting 

 conclusions drawn from the study of paleontological series/^ 

 The conclusions of Osborn are based on his extensive 

 studies of the fossil collections of the American Museum of 

 Natural History, and elsewhere, the details of which are 

 published in voluminous memoirs. His recent conclusions 

 have been based largely on study of evolutionary series in the 

 elephants and their relatives. Some of the essential data and 

 conclusions may be sketched as follows: 



1. Structures ("aristogenes") that in later ages become con- 

 spicuous and functionally important appear at the beginning 

 as almost imperceptible features; so slight that their func- 

 tional use is at this time doubtful. Classical examples of these 

 things are the cones on the teeth of various vertebrates. They 

 arise as almost imperceptible prominences (see the figures in 

 the papers of Osborn above referred to). 



2. In succeeding ages these structures increase in promi- 

 nence and functional value in a continuous way without sud- 

 den changes. Continuity, not discontinuity, is the characteris- 

 tic of such evolutionary change. 



3. The structures follow a well marked definite course, in 

 their changes from the imperceptible, doubtfully functional, 

 condition, to the conspicuous and useful organs that they 

 finally produce. Such definite change following a course 

 toward a well defined useful end is spoken of at times as 

 orthogenesis, or rectigradation. 



4. In their later condition the structures produced are useful 

 in a high degree. 



5. The same types of structures (e.g. grinding cones on the 

 teeth) arise independently in different divisions of the same 

 group of organisms, so that they are said to be "predetermined 

 in the germ plasm" of the group. Yet other divisions of the 

 same group do not produce these structures, but others of a 



