No. 2.] THE HARD PARTS OF THE MAMMALIA. 201 
change in the distribution and determination of growth-energy 
in the embryo. This sustains the dynamic theory of heredity 
which I proposed in 1871, and which was subsequently termed 
perigenesis by Haeckel. Sucha hypothesis is equally necessary 
to explain the transmission of characters, which are more or less 
developed in the adult by the mechanical energy of impacts and 
strains. 
B. The Muscular Insertions. 
As regards muscular insertions, the best marked are those 
for the muscles supporting the head on the vertebral column, 
and for moving the lower jaw in mastication. Strength of the 
former has developed the strong occipital crest of the inion. 
Size of the temporal muscle determines the expanse of the 
gygomata, and the presence or absence of a sagittal crest. 
Forms which move the lower jaw transversely have the tem- 
poral muscles inversely as the extent of the lateral excursions 
of the jaw. Hence these muscles have a diminished size in 
such forms as the ruminants, and are widely separated. On the 
other hand, the pterygoid muscles and their osseous insertions 
are enlarged. This is still more the case with forms which have 
an anteroposterior movement of the lower jaw, as the Rodentia 
and Proboscidea, where the pterygoid fossa is very large. 
The position of the dental series with reference to the skull 
in general has reference to the energy of the action of the 
temporal and masseter muscles. In primitive Mammalia a con- 
siderable part of the molar series is below and posterior to the 
position of the orbit, and the series has been prolonged poste- 
riorly in forms which possess the proal or palinal mastication, as 
Rodentia and Proboscidea. In forms with ectal or ental masti- 
cation, on the other hand, the molar series has gradually moved 
forwards so as to be entirely anterior to the vertical line of’ the 
orbit, as in the horse and the Bovide. The camel retains the 
primitive relation in this respect.1 This apparent anomaly may 
be explained by the fact that the camel retains large and pow- 
erful canine teeth, which he uses as weapons of offence and 
defence, so that his temporal muscles retain the large develop- 
ment of the early forms. This is further indicated by the 
1 See Kowalevsky Monographie d. Anthracotherium, 1873, Pl. IX. 
