Chap IV. 
MANNER OF DEVELOPMENT. 
121 
increased or decreased use of parts, the facts now given 
shew that his liability in this respect has not been lost ; 
and we positively know that the same law holds good 
with the lower animals. Consequently we may infer, 
that when at a remote epoch the progenitors of man 
were in a transitional state, and were changing from 
quadrupeds into bipeds, natural selection would probably 
have been greatly aided by the inherited effects of the 
increased or diminished use of the different parts of the 
body. 
Arrests of Development . — Arrested development differs 
from arrested growth, as parts in the former state con- 
tinue to grow whilst still retaining their early condition. 
Various monstrosities come under this head, and some 
are known to be occasionally inherited, as a cleft-palate. 
It will suffice for our purpose to refer to the arrested 
brain-development of microcephalous idiots, as described 
in Vogt’s great memoir . 34 Their skulls are smaller, and 
the convolutions of the brain are less complex than in 
normal men. The frontal sinus, or the projection over 
the eye-brows, is largely developed, and the jaws are 
prognathous to an “ ejfrctyant ” degree ; so that these idiots 
somewhat resemble the lower types of mankind. Their 
intelligence and most of their mental faculties are 
extremely feeble. They cannot acquire the power of 
speech, and are wholly incapable of prolonged attention, 
but are much given to imitation. They are strong and 
remarkably active, continually gamboling and jumping 
about, and making grimaces. They often ascend stairs 
on all-fours ; and are curiously fond of climbing up 
furniture or trees. We are thus reminded of the delight 
34 ‘ Memoire sur les Microcephales,’ 1867, p. 50, 125, 169, 171, 181- 
198, 
