1892,] The Difficulties in the Heredity Theory. 551 
by the establishment of the major axis through the large toe 
and atrophy of the outer toes. The present atrophy of our 
small toe is as good a parallel as we can find of the changes 
which were occurring in the eocene period among the ances- 
tors of the horse. 
The Teeth—But how about the teeth, in which there is an 
absolute loss of tissue in consequence of use? This is another 
objection raised by Ball, Poulton, and others which disappears 
upon examination. 
The dental tissues, while the hardest in the body, and, 
unlike bone, incapable of self-repair, are not only both living 
and sensitive, but, to a very limited degree, plastic and capa- 
R.—frd, protoconid (anterior buccal 
ial cusp): hyd, hypoconid (posterior 
t i Fig. 1.— 
ig. ing of ancestral cusps 
Fig. 6.—Eocene carnivore = Eocene lar crown s, ved 
high crown mz. Fig. 7.— 
lingual cusp, fad, disappears. 
ble of change of form. Ex hypothesi, it is not the growth, but 
the reaction tendency which produces the growth, which is 
transmitted. The evolution of the teeth, therefore, falls into 
the same category as bone.’ In the accompanying figures I 
‘See especially the papers of Ryder, Cope, and the writer, “ Evolution of Mammal- 
ian Molars to and from the Tritubercular Type,” American Naturalist, 
