166 The Descent <>f Man. 



have resulted accordingly. The ])ar\viuian hypothesis ex- 

 cludes this important factor from the explanation of organic 

 evolution. Natural selection proper does not account for 

 the origin of intelligence, nor of design in structure; it 

 only accounts for direction or tendency. Darwin's theory 

 is true, nevertheless, as far as it goes. It accounts for the 

 destruction of certain peculiarities, and for consequent 

 striu'tural and functional modifications, but it does not ex- 

 plain the origin of variations. 'JMius variation and natural 

 selection combine to produce the results which we see in 

 evolution. 



We have obtained in North America partly complete 

 genealogies of most of the modern types of Mammalia, but 

 these do not include man, although his earlier ancestors, 

 the lemurs, are abundantly represented. The lines of the 

 deer, camel, peccary, tapir, horse, rhinoceros, cat, dog, beaver, 

 squirrel, and various other mammalian types are very com- 

 l)lete. From their fossil skeletons we can trace the changes 

 which have successively appeared during geological time, 

 and can investigate the causes which have produced them. 

 After a careful survey of the field I am of the opinion that 

 the origin of new types in the hard parts can be ascribed to 

 the interaction of mechanical impacts and strains, Avith the 

 antagonistic force of persistence of type, ami that Avhenever 

 the former overcomes the latter, a new structure appears, 

 which is transmitted sooner or later to the succeeding genera- 

 tions. The evidence for this belief is abundant, and in some 

 cases very simple ; in other cases it is more complex. This 

 theory explains the exact adaptation of animal mechanisms 

 to their uses, and renders clear the continued progress in the 

 direction of mechanical perfection which we find to have 

 taken place throughout the ages of geological time. An 

 opposing theory (that of Weismann) holds that acquired 

 characters cannot be inherited, but does not attempt to ex- 

 plain the origin of variation. The evidence of paheontology 

 indicates that acquired characters have been inherited, though 

 exactly how this has been done requires explanation. 



Sensation exists in the lowest forms of animal life, and 

 it is accompanied by memory. Sensation and memory ex- 

 press the simplest acts of consciousness. A sensation ex- 

 perienced is likely to be recalled by association. Even the 

 lowest animals show discrimination in the selection of food, 

 appropriating that which will nourish them and rejecting 



