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 

 bodv. 



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 " ejfrayant " 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 tip 

 furniture or trees. We are thus reminded of the delight 



34 'Mernoire sur les Microcephales,' 18(37, p. 50, 125, l^lYl^.isi- 

 198. /^ i* -*>' 



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