THE LAWS OF EVOLUTION EMPHASIZED. 363 



pie in the evolution of the forefoot of mammals, as shown in 

 the figure on the opposite page. 



Finger-bones and Teeth as Tests of Degree of Differentiation. — In 

 tracing the history of mammals \vc find the principle of five 

 fingers already developed before mammals began. Hence 

 the wonderful modifications noted by Owen, Kowalevsky, 

 Ryder, Marsh, Cope, and others, in the arrangement of the 

 bones of the mammalian feet, their specialization in form, and 

 relative size, shape, and position, have constituted the chief 

 data for both classification and phylogenetic series. 



The teeth, as highly specialized organs, and as terminal 

 parts of the individual organization, coming into most im- 

 mediate contact with the outside elements of resistance to 

 the life of the individual, are particularly sensitive expres- 

 sions of the stages of evolution. 



Any device of offence or defence, particularly when hard- 

 ness and resistance to attrition are characteristics of its struc- 

 ture, becomes at once a mark of the effects of environment 

 in inducing modifications, and of the stage of progress 

 attained by the individuals in their evolution. Their resist- 

 ance to destruction makes such parts most valuable records 

 in the rocks of the history of organisms. 



Laws Derived from the Study of the Teeth of Mammals by 

 Osborne. — Professor H. F. Osborne, following the investigations 

 of Riitemeyer and others, has recently written several instruc- 

 tive papers setting forth the laws to be observed in the 

 histor)'' of the development of the teeth in mammals. 



In a memoir (first read as the address of the vice-president 

 of the section of Zoology, of the American Association for the 

 Advancement of Science*) he narrates both concisely and 

 admirably the laws expressed in the modification of the cusps 

 or surface forms of the teeth of mammals. 



Osborne shows how the tricuspid tooth is an evolution 

 from a simple monocuspid tooth, which is the primitive type 

 of tooth in all earlier vertebrates. He shows further that the 

 multiple succession of teeth characteristic of reptiles is the 

 primitive method of arrangement, and this, as is also the in- 



* " The Rise of the Mammalia in North America," Am. Jour. Set., 

 III., vol. XLVi., pp. 379-392 and 44S-466. 



