Il6 ITNDERHILI. : 



apples alone, but was a universal one. And so if evolution 

 had been demonstrated only by what we know as to the origin 

 of the horse, this would be sufficient to establish it as universal 

 in its application. In America, where the record is most 

 complete, the horse has been traced from the Mammalian 

 Dawn down to historic times through eight successive stages 

 without a break of importance in the line, and, as we study 

 these steps' that lead up to the most highly specialized of 

 modern animals, with their accompanying phenomena of envi- 

 ronment, we are furnished with the best exemplification in 

 existence of the laws which govern animal evolution. 



It is of course not to be inferred that the distinguishing 

 characteristics in this series were abrupt departures from pre- 

 vailing forms : the horse with one functional digit between 

 two that were useless was not a direct product of the preceding 

 genus in which the two lateral hoofs reached the ground and 

 contributed to support, nor did this three hoofed horse thus 

 descend from the one which stood upon four. Between these 

 stages there was an intermediate series in which the tendency 

 to discard what had become an incumbrance went hand in 

 hand with adaptive development. If a variation is of advan- 

 tage ill the struggle to sustain life, nature tends to retain it 

 and intensify it in future generations until it finally predomi- 

 nates over the older forms. To be sure nature also tends to 

 propagate defects, l)ut this retrogression cannot be long sus- 

 tained amid constantly unfolding enemies to life, and, as the 

 contest becomes sharper and the range more restricted, only 

 those responsive to the change can survive. It is a clear 

 applicati(^n of Spencer's " Survival of the Fittest." 



The significance of these variations as factors in the evo- 

 lution of species can be better appreciated if we consider the 

 vast stretches of time in which they were having their influ- 

 ence. As an example we may take the case of the so-called 

 wolf-tooth of our present day horses. In the ancestral types 

 this first of the four premolars was fully developed and had its 

 opposing tooth in the lower jaw. As the lower tooth was the 



