JENNINGS: CHANGES IN HEREDITARY CHARACTERS 297 



that appear to modify seriously, if they do not quite nullify, 

 this conclusion that variations follow a determinate course. 



First, we do find that diverse courses are followed by given 

 characters, in diverse branches of a given group; this is partic- 

 ularly true of the characters of shape and proportion, which 

 Osborn calls allometrons. I take it from the descriptions that 

 this is likewise true at times for structural and numerical 

 characters. 



A second point which Osborn sets forth is deserving of partic- 

 ular attention. He states, in agreement with Waagen, that 

 in any given geologic stratum, we do find, in addition to char- 

 acteristics that are in the line of determinate descent, other 

 variations from this line, which are of the sort that constitute 

 what we call at the present time varieties; things that are like 

 the diverse races of DifHugia in my own work. But, say Os- 

 born and Waagen, there is a great difference in principle between 

 these and the others, for those which are in the determinate line 

 of progress persist into the next geologic stratum, while the 

 mere varieties do not. The persistent changes were called by 

 Waagen, mutations (in a sense somewhat diverse from that in 

 which the word is used by de Vries). 



Osborn expresses the opinion that these "varieties" may be 

 merely non-heritable modifications." But in our present geo- 

 logic period we find just such diverging forms, in great number, 

 and we find that their peculiarities are heritable; this I empha- 

 sized in the introductory part of the present discussion. There 

 is then no reason for supposing that these variations were not 

 heritable in earlier geologic periods; there must have been many 

 races heritably diverse, just as there are now; and these are what 

 Waagen called varieties. 



Now since this is so, the only difference between Waagen's 

 mutations and his varieties, is that, on looking backward at 

 them, we find that the former persisted and the latter did not. 

 But this tells us nothing whatever about why the latter did not. 

 It is perfectly possible, so far as these facts go, that it was a 



11 Osborn, 1915, p. 225. (See Bibliography.) 



