DARWIN AND PALEONTOLOGY Ml 



these branches appear to have chosen their own 

 local environments, whether in localities favor- 

 able to grazing or to browsing, and in turn con- 

 genital changes of proportion would be favored 

 by selection if in the right direction. The trans- 

 formation into brachycephaly and doUchocephaly 

 is brought about through independent changes of 

 proportion in every bone of the skull, as ascer- 

 tained by exact comparative measurements. A 

 trend once established in either direction seems to 

 constitute a sort of " hereditary momentum " or 

 predisposition, which leads to great extremes of 

 brachycephaly, on the one hand, or dolichoceph- 

 aly on the other, as shown in the accompanying 

 cut. The rudimentary horns, at first barely no- 

 ticeable as the faintest convexities of the skull 

 invariably appearing at the junction of the 

 frontals and nasals, and produced by a thicken- 

 ing of the cellular spaces, are first observed of 

 equal size in the males and females; later they 

 become more prominent in the males than in the 

 females; finally they assume vast proportions in 

 the males and present an arrested development 

 in the females. At the summit of the Eocene 

 the extreme dolichocephalic and brachycephalic 

 phyla die out, and in the Oligocene a new series 

 of phyla arise. Among these the long-horned 

 forms appear through selection to develop the 

 horns at the expense of other characters, the 

 males with the longest horns probably securing 

 the most females and becoming the chief breed- 



