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THE AMERICAN NATURALIST [Vol. XLIX 



pears to break down these separable allometrons and ul- 

 timately results in blending. This may be partly due to 

 the fact that changes of cranial proportion occur not only 

 within the species, but within the races and sub-races of 

 Homo sapiens, as witnessed in the mongoloid Indian 

 races of North and South America. In other words, the 

 allometrons in man are of more recent origin than in the 

 horse and ass, which probably separated from each other 

 as far back as the Lower Miocene. Further experiments 

 and observations are greatly needed as to the separable- 

 ness or blending of allometrons in hybrids. 



As to the rapidity of evolution of proportional and 

 numerical characters it appears that in certain lines al- 

 lometrons may evolve more rapidly than rectigradations. 

 This is seen in the titanotheres (Figs. 4, 5, 10), in which 

 changes of proportion develop very rapidly, while the 

 rectigradations on the grinding teeth and the rudiments 

 of horns develop very slowly. On the other hand, in the 

 contemporary Eocene horses the rectigradations seen in 

 the addition of cusps develop very much more rapidly 

 than the changes of proportion in the skull. This con- 

 trast between horses and titanotheres, however, confirms 

 the universal law that every "character" has its differen- 

 tial phyletic movement as well as its differential onto- 

 genetic movement. 



That these movements are not identical is further 

 shown by a familiar illustration. The median toes of 

 the feet of the desert-living Hipparion have a much more 

 rapid phyletic movement than the median toes in the 

 forest-living Ihjpohlppus, yet we may be sure that the 

 limbs of the newly born foals of Hipparion and of 

 Tljlpohlppus were alike relatively elongated to enable 

 these foals to accompany the mares in flight, this adapta- 

 tion being secured through ontogenetic movement, or ac- 



These differentials in the velocity of characters in their 

 phyletic and in ontogenetic movements may afford one 

 of several reasons why allometrons, or proportional 

 characters are separable in hybrids, why some "unit 



