84 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
Significance of the descent from trees.—As a result of the descent 
from the trees, certain definite factors were called into play, each of 
which had its effect on the further evolution. Briefly enumerated, 
these are: (1) Assumption of the erect posture; (2) liberation of the 
hands from their ancient locomotor function to become organs of the 
mind; (3) loss of the easily obtainable food of the tropical forests, 
necessitating the search for sustenance, both plant and animal, and 
man became a hunter; (4) need of clothing with increasing inclemency 
of the weather, especially during the long winters; (5) freedom from 
climatic restrictions—when an omnivorous diet and clothing were 
acquired man was no longer limited to one definite habitat and the 
result was dispersal; (6) the development of communal life, rendered 
possible by the terrestrial habitat. Primates are at best gregarious, 
submitting, as in the gorilla, to the leadership of the strongest male, 
but it is only by communal life with its attendant division of labor 
that man can rise above the level of utter savagery. 
Evolutionary changes.—Human evolutionary changes which are 
recorded are: more erect posture, shorter arms, perfection of 
thumb opposability, reduction of muzzle and of size of teeth, loss 
of jaw power, development of chin prominence, increase in skull 
capacity, diminution of brow-ridges, diminution in strength of zygo- 
matic or temporal arch, increase in size and complexity of brain, 
especially frontal lobes, development of articulate speech. 
FOSSIL MAN 
Fossil remains of man are found: under two conditions, in river 
valley deposits and in limestone caverns which served first as a 
dwelling-place and later as a sepulture. Of these the caverns 
have been by far the most productive, but they contain only the 
remains of the later races, as the caverns according to Penck did not 
become available for human occupancy before middle Pleistocene 
time. 
The rarity of human fossils may be explained, first, by the various 
burial customs which seldom are sufficiently perfect to preclude the 
possibility of alternate wetting and drying or of rapid oxidation, both 
of which are prohibitive of fossilization. If man lived and died in the 
forests the chances for his fossilization, in common with other forest 
creatures, was very remote, for the remains of such are almost invari- 
ably destroyed by other animals, by dampness, or by fungi, and rarely 
attain a natural burial in sediment. If, on the other hand, he dwelt 
