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THE AMERICAN NATURALIST [Vol. XL VI 



1. Skull and horns of titanotheres (Figs. 1, 3, 4). 



2. The horns of cattle (Fig. 2). 



3. The cranium of man (Fig. 1). 



4. The skull of horses (Figs. 4, 5, 6, 7). 



5. Teeth (Fig. 8). 



One of the most salient examples of the genesis of 

 unit characters through continuity is that of the evolu- 

 tion of horns, i. e., of the osseous prominences on the 

 skull. Horns are now known definitely to be "unit char- 

 acters," first through their sudden and complete disap- 

 pearance in the niata and polled breeds of cattle ; second, 

 because they conform to the laws of sex-limited inherit- 

 ance. The question is, do horns originate continuously 

 or discontinuouslv? 







1. Horns of Titanotheres 

 The titanotheres are an extinct family of quadrupeds 

 distantly related to the horses, tapirs and rhinoceroses, 



