400 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



not modified the shape of that part of the structure in 

 the race. I have myself cut off the tails of nine stic- 

 cessive generations of mice without producing the 

 slightest effect on the length of the tails of the tenth. 



Nevertheless such negative evidence only demon- 

 strates that such modifications of the structure may 

 not be inherited. A single undoubted example of the 

 inheritance of a mutilation would prove that no in- 

 surmountable barrier to such inheritance exists. And 

 well authenticated examples of such cases are known 

 and will be mentioned later on. But it is not with 

 mutilations that the paleontologist has to do. The 

 rupture of the hymen and circumcision, and most muti- 

 lations, can only occur once in the life of the individual, 

 and generally they produce no appreciable direct effect 

 on his or her metabolic physiology. Moreover, the 

 mutilations above cited as not inherited are experi- 

 enced by but one sex, except in the case of the tails 

 of the mice. The question is widely different with re- 

 gard to the parts of the structure in which we observe 

 the real differences between organic types. The defi- 

 nitions of natural divisions rest to a great extent on 

 the diversities displayed by the organs of motion and 

 nutrition. Now these are in use in animals during 

 most of the hours not spent in sleep. Their move- 

 ments are perpetual, and their activities only cease 

 with death. It is then quite unreasonable to cite the 

 history of mutilations as evidence against the inheri- 

 tance of natural characters produced by oft-repeated 

 and long continued natural causes. 



It has been shown in Part Second of this book that 

 structural characters are produced by use and other 

 stimuli to growth. It has also been shown in Part 

 First that the characters so produced show a progres- 



