276 



THE AMEBIC AX XA TURAL1ST 



Vol. XLVI 



This evidence, in the opinion of T. H. Morgan, is in 

 entire accord with the modern views of hybridizing; 

 parallels for each instance can be given; without the 

 evidence of the F 2 generation no conclusions adverse 

 to Mendelism are possible. Even the differences in re- 

 ciprocal crosses, i. e., horse J, ass $, can be understood 

 if sex-limited inheritance prevails in some characters. 



To confirm the results suggested by this Fj genera- 

 tion of the horse and ass, it would be necessary to inter- 

 breed races of mammals to F 2 or F 3 to ascertain whether 

 these characters of the skull and teeth really mendelize. 

 It is doubtful whether such specific types of mammals can 

 be found, and whether sufficient stability of character 

 exists in artificially produced races. 



Sufficient evidence has been adduced, however, to show 

 that a very large number of characters which are to the 

 best of our knowledge of continuous origin, present all 

 the appearance of "unit characters" in the first genera- 

 tion of hybrids. 



III. Conclusion 



Is it not demonstrated by this comparison of results 

 obtained in such widely different families as the Bovidae, 

 Hominidae, Titanotheriidae and Equidae that discontinu- 

 ity in heredity affords no evidence whatever of discon- 

 tinuity of origin? 



As to origin, it is demonstrated in paleontology that 

 certain new characters arise by excessively fine grada- 

 tions which appear to be continuous. If discontinuities 

 or steps exist they are so minute in these characters as to 

 be indistinguishable from those fluctuations around a 

 mean which seem to accompany every stage in the evolu- 

 tion and ontogeny of unit characters. 



IV. Theoretical Considerations 

 After having attempted to confine our discourse to 

 facts it is a pleasure to relax into the more genial at- 

 mosphere of opinion and hypothesis. 



