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THE AMEBIC AX NATURALIST [Vol. XL VI 



(Rangifer) and the hartebeest (Bubalis). Cytocephaly 

 is an ontogenetic and phylogenetic new character, aris- 

 ing or developing continuously. 



As in the case of the human skull; the causes of these 

 profound changes in head form are entirely unknown; 

 the mechanically adaptive significance is sometimes ap- 

 parent, sometimes obscure. The evidence is strengthened 

 by the examination of the titanotheres that human selec- 

 tion has little or no influence on human cranial form. 

 The great point to emphasize is that this allometric evo- 

 lution in the skull and all parts of the skeleton is the pre- 

 vailing phenomenon of change in the skeleton of mammals. 

 It is constantly in progress and is universally, so far as 

 we can observe, a continuous process. As displayed in the 

 four phyla of titanotheres (Fig. 3), the elongation or 

 broadening of the foot bones proceed independently and 

 are divergent, while in the same mammals the rectigra- 

 dations exhibited in the rise of similar cusplets on the 

 teeth and similar horn rudiments on the face are parallel; 

 in the former case no ancestral predisposition seems to 

 be operating, in the latter case ancestral predisposition 

 certainly seems to operate ; this is why the internal laws 

 controlling the origin of new allometrons and of new 

 rectigradations and allometrons are regarded as essen- 

 tially dissimilar. 



Paleontological analysis of these rectigradations and 

 allometrons even unaided by experimental heredity re- 

 veals the essential feature of the "unit character" prin- 

 ciple, namely, that what ice arc observing is. an incredibly 

 large number of unit elements each of which enjoys a 

 certain independence of evolution at the same time that 

 each unit is adaptively correlated with all the others. For 

 example, in the upper and lower grinding teeth of horses 

 alone there are 504 cusp units, each of which has an inde- 

 pendent origin and development ; at the same time each 

 cusp is more or less distinctly correlated in form with 

 the all-pervading dolichocephaly or brachycephaly of the 

 skull ; in fact, from certain single cusps of the teeth we 



