252 THE ORIGIN AND EVOLUTION OF LIFE 



in the changes of form and proportion in evolution as in 

 growth. Second, the law of rectigradation, under which many 

 important new characters appear definitely and take an adap- 

 tive direction from the start; third, the law of acceleration and 

 retardation, witnessed both in racial and individual develop- 

 ment, whereby each character has its own velocity, or rate of 

 development, which displays itself both in the time of its origin, 

 in its rate of evolution, and its rate of individual development. 

 This last law underlies the profound changes of proportion in 

 the head and different parts of the body and limbs which are 

 among the dominant features of mammalian evolution. In 

 the skeleton of mammals very few new characters originate; 

 most of the changes are in the loss of characters and in the 

 profound changes of proportion. For example, by the addi- 

 tion of many teeth and by stretching or pulling, swelling or 

 contracting, the skeleton of a tree shrew may almost be trans- 

 formed into that of a whale. 



The above laws are the controlling ones and make up four- 

 fifths of mammalian evolution in the hard parts of the body. 

 So far as has been observed the remaining fifth or even a 

 much smaller fraction of mammalian evolution is attributable 

 to the law of saltation, or discontinuity, namely, to the sudden 

 appearance of new characters and new functions in the hered- 

 ity-chromatin. For example, the sudden addition of a new 

 vertebra or vertebrae to the backbone, which gives rise to the 

 varied vertebral formulae in different orders and even the dif- 

 ferent genera of mammals, or the sudden addition of a new 

 tooth are instances of saltatory evolution in the hard parts 

 of the body. There are also many instances of the sudden 

 appearance of new functional, physiological, or physicochem- 

 ical characters, such as immunity or non-immunity to certain 

 diseases. 



