268 THE ORIGIN AND EVOLUTION OF LIFE 



to believe that this animal, like the existing okapi, was protected 

 by coloration and by its swamp-living habits. 



The above examples illustrate the general fact that changes 

 of proportion make up the larger part of mammalian evolution 

 and adaptation. The gain and loss of parts, the presence and 

 absence of parts, which is so conspicuous a phenomenon in 

 heredity as studied from the Mendelian standpoint, is a com- 

 paratively rare phenomenon. These changes of proportion are 

 brought about through the greater or less velocity of single 

 characters and of groups of characters; for example, the trans- 

 formation of the four-toed horse of the base of the Lower 

 Eocene^ into the three-toed embryo of the modern horse is 

 brought about by the acceleration of the central digit and the 

 retardation of the side digits. This process is so gradual that 

 it required 1,000,000 years to accomplish the reduction of the 

 fifth digit, which left the originally tetradactyl horse in the 

 tridactyl stage (Fig. 130); and it has required 2,000,000 years 

 more to complete the retardation of the second and fourth 

 digits, which are still retained in the chromatin and develop 

 side by side with the third digit for many months during the 

 early intrauterine life of the horse. 



No form of sudden change of character (saltation, muta- 

 tion of de Vries) or of the chance theory of evolution (pp. 7, 8) 

 accounts for such precise steps in mechanical adjustment; be- 

 cause for all proportional changes, which make up ninety-five per 

 cent of mammalian evolution, we must seek a similar cause, 

 namely, the cause of acceleration, balance or persistence, and 

 retardation. This cause may prove to be in the nature of phys- 

 icochemical interactions (p. 71) regulated by selection. The 

 great importance of selection in the evolution of proportion is 



^The earliest-known fossil horses are four-toed, having lost the first digit (thumb). 

 No five-toed fossil horse has yet been found. 



