350 



ANIMAL LIFE 



in nature to which they were subjected. Thus, by varia- 

 tion and natural selection numerous and widely different 

 kinds of animals arose. 



Variation in the individuals of certain species, and there- 

 fore evolution, is occurring at the present time, but much 



Fro. 392. The four-toed horse restored from a study of its fossil skeleton. From 

 Guide Leaflet of the Museum of Natural History, New York. 



more slowly than during the world's infancy, when climate, 

 food, and other factors were changing more rapidly. An 

 observer, seeing for the first time the very dark and very 

 light forms only of the Catalpa caterpillar, would at a 

 glance declare them two different species, but after find- 

 ing specimens with all the gradations in shade from light 

 to dark (Fig. 394), he would be forced to consider the two 

 unlike individuals to belong to the same species. If, in the 

 future, the four intervening forms shown in the figure be- 

 come extinct and the adults of number one and number 

 six differ as widely as the larvse and remain constant in 



