50 SOCIAL EVOLUTION 



We have now reviewed the evidence which leads us 

 to believe that man is related to forms of life still ex- 

 tant. This evidence constitutes a presumption which 

 justifies us in the belief that we shall discover the inter- 

 mediate forms and so partially complete the series of 

 man's descent. The gaps in this series must be filled 

 by the reconstructed skeletons of bone remains of pre- 

 historic man. In consideration of the fact that bone 

 usually decays within a comparatively short time, the 

 chances are slight of finding remains in a sufficient state 

 of preservation to constitute positive evidence. Under 

 certain peculiar conditions, these bone remains are pre- 

 served for great periods of time. They must be pro- 

 tected from the action of the air, the corrosive action of 

 water, and from the destructive action of insects and cer- 

 tain plant agencies which cause decay. The necessary 

 conditions are present in dry caves and where natural 

 agencies have deposited layers of sand and gravel. 

 Hence it is that the bone remains of prehistoric man 

 are most frequently found in undisturbed boulder-clays 

 and drift, or imbedded in the floor of some cave of great 

 antiquity. Investigation and discovery have brought to 

 light a considerable number of bone remains of prehis- 

 toric man. We shall, therefore, examine this evidence 

 to ascertain how far we may expect to reconstruct the 

 intermediate steps in the descent of man. 



In the first place how are we able to tell certainly 

 whether any bone remains which we find are reminis- 

 cent of prehistoric man? Is it not possible that they 

 are simply the remains of some relatively modern patho- 

 logical individual and not of some lower type of man? 

 The associated circumstances are of utmost importance. 

 If the particular part of a skull which we have discovered 



