47 
unsymmetiy varies, it is probable that no such thing as a 
perfectly symmetrical human skull exists. 
As to orthognathism and prognathism, it may be observed 
that Retzius includes amongst the orthognathi the Celtic 
Scotch, Irish, and Welsh. Any one who has travelled 
amongst these peoples would be able to confute the uni- 
versal, or even general, truth of this statement. Amongst 
the lower Irish, indeed, prognathism is the prevailing type, 
and there is this further interest about the subject, that 
prognathism appears to be a type rapidly acquired by 
changed external circumstances. The conclusions arrived 
at are as follows : — 
It is probable that when the struggle for existence was 
less keen than it is at present, and the human brain was in 
consequence less prone to rapid growth, human skulls pre- 
served a pretty uniform type, thus, e.g., all the neolothic 
skulls yet found are dolicocephalic, and what is also worth 
noting, they are of an unusually symmetrical character. It 
is in accordance with the doctrine of evolution to suppose 
that different environments (such as differences in climates, 
soil, mode of livelihood, e.g., living by the chase or by agri- 
culture) would produce certain and definite cranial changes : 
hence would arise national types of skulls, slow in arriving 
at such a difference as exists between the Eskimo and the 
Negro, and slow in changing that type when acquired. 
After a time the influence of civilization would come into 
operation, which would tend to produce varieties in the 
crania of a nation in accordance with the varieties of the 
environments of the individuals comprising the nation. A 
similarity of external circumstances and an absence of 
intermarriage would tend to produce but one type of 
skull, a difference in external circumstances and inter- 
marriage would tend to produce a varying type. These 
factors are both at work in civilized countries. Nations 
whose skulls have long ago been of a well-marked distinctive 
character are exposed to the same environments and inter- 
marry — the result is a confusion and mingling of the 
different forms. 
