No. 545] ORIGIN OF UNIT CHARACTERS 



2H3 



allometric change strictly comparable to that which oc- 

 curs in the crania of lower mammals, especially as ob- 

 served in the horses and titanotheres ; the extremes are 

 produced in so-called pure human races under geographic 

 isolation; when these pure races are brought together 

 there arises disharmonism or alternating heredity or 

 both. Neither the dolichocephalic nor brachycephalir 

 type is as yet known to be dominant ; opinion is divided 

 as to whether in the first cross the heredity is pure or 

 whether there may be a tendency to produce an inter- 

 mediate form; opinion is nearly unanimous that pro- 

 longed interbreeding produces blends. 36 



4. Skull of Titanotheres. 

 The continuity of allometric evolution in the skull of 

 the titanotheres (Fig. 4) has been the subject of pro- 

 longed investigation by the writer, assisted by Dr. W. K. 

 Gregory, involving thousands of measurements, many of 

 which belong in strictly successive phyletic series. 

 Allometry (i. e., the measurement of allometrons) here 

 applies to the skull as a whole. We secure the cephalic 

 index by dividing the breadth across the cheek arches by 

 the total basilar length of the skull. There are also other 

 indices, such as the facio-cranial, in which we measure 

 continuous trends of allometric change; brachycephaly 

 and dolichocephaly arise independently in four different 

 phyla or lines of descent. The adaptive significance is 

 sometimes apparent, sometimes obscure. As shown in 

 Fig. 1 the titanotheres, like man, exhibit facial abbrevia- 

 tion and cranial elongation (postopic dolichocephaly) in 

 contrast with the facial elongation (proopic dolicho- 

 cephaly) of the horses. These phenomena are similar 

 to those of cytocephaly, or the bending down of the face 

 upon the base of the cranium as observed in the reindeer 



