98 THE RE1[:>AXT TEETH. 



The article closes as follows: 



''Such is, in brief, a general outline of the more 

 marked changes that appear to have produced in 

 America the highly specialized modern Equus from 

 its diminutive, four-toed predecessor, the Eocene Oro- 

 hippus. The line of descent appears to have been 

 direct, and the remains now known supply every im- 

 portant intermediate form. Considering the remark- 

 able development of the group throughout the entire 

 tertiary period, and its existence even later, it seems 

 very strange that none of the species should have sur- 

 vived, and that we are indebted for our present horse 

 to the Old World."* 



* The follo^viag extracts from Prof. C. S. Tomes's "Dental 

 Anatomy, Human and Comparative" (pp. 247-8, 254-5), explain 

 some of the causes of the raetamorphos.^s described by Prof. 

 Marsh ; " He would indeed be a rash man who ventured to as- 

 sert that we had recognized all the agencies which are at work 

 in the modeling of animal and vegetable forms ; but it is safe to 

 say that, at the present time, we are acquainted vnth several 

 agencies which are in constant operation, and which are com- 

 petent to profoundly modify animals in successive generations. 

 We know of 'uaUiral selection/ or 'survival of the fittest,' an 

 agency by which variations beneficial to their possessors will be 

 preserved and intensified in successive generations ; of * sexual 

 selection,' which operates principally by enabling those pos- 

 Sf^ssed of certain characters to propagate their race, while others 

 less favored do not get the opportunity of so doing-; of 'con- 

 comitant variation' between different parts of the body, an 

 agency much more recondite in its operations, but by which 

 agencies affecting one part may secondarily bring about altera- 

 tions in some other part. 



"The doctrine of natural selection, or survival of the fittest, 

 is as applicable to the teeth of an animal as to any part of its 

 organization, and the operation of this natural law will be con- 

 stantly tending to produce advantages or 'adaptive' differences. 

 On the other hand, tlie strong power of inheritance is tending to 



