No. 1.] TRITUBERCULAR MOLAR. II 
be found in the following tables. But it must be remembered 
that it is not always possible to determine exactly the race of 
the person represented by a skull, even when care in its identi- 
fication has been exercised. Emigration and war have con- 
stantly rendered races impure, and transplantation on a large 
scale has in some parts of the earth produced hybrid races. The 
results of a study of human crania are sure to be more or less 
vitiated by these circumstances. We obtain averages rather 
than exact definitions. Nevertheless, the extremes of the series 
of variations are likely to be found to be characteristic of estab- 
lished forms of man, and will thus justify my belief in the value 
of the characters presented. To ascertain the relation of these 
variations to the races is the object of the present inquiry. 
The cause of the tritubercular reversion belongs to the class 
of agencies active in evolution of organic types, of whose real 
nature we know little. It cannot be said to be due to a con- 
traction of the maxillary arcade, for the Esquimaux and some 
other peoples which display the tritubercular dentition are not 
deficient in this respect. Nor do tritubercular molars require 
less space than the quadritubercular, for the external width of 
the crown is the same in both cases. They generally require 
less material however than a quadritubercular crown, since a 
triangle is smaller than a square drawn on the same base line; 
however, in some men of the lower races who present the tritu- 
bercular molars, their outline is nearly square. The hypothesis 
advanced to account for the reduction of the number and quality 
of human teeth observed in the higher races, as well as for the 
replacement of the prognathous jaw by the orthognathous, is 
that such changes are due to a transference of material and of 
growth energy from these parts to the superior part of the skull 
and its contents. The relative superiority of the dimensions of 
these parts in the higher races is thus accounted for. 
In the following tables the tubercular formule are represented 
by numbers. Only the last three, or the true molars, in each 
jaw are considered. Tubercles of reduced size are represented 
by fractions. Thus eae indicates that each superior molar 
is quadritubercular, and each inferior molar quinquetubercular. 
This represents the extreme of the series represented by the 
4a 3 
A 44 
lowest races. The formula 
indicates that the true molars 
