CHAPTER XVIII 



DEVELOPMENTAL LINES AND TREES- 

 EVOLUTION 



THE first of the three sets of facts to be con- 

 sidered in connection with the development of 

 animal forms has to do with the existence 

 within each of the larger of the so-called phyla or 

 major groups of animals of well marked, obvious and 

 undeniable evolutionary lines which, having their 

 origin in far distant geologic time in a relatively 

 simple form of creature, run by easy stages to a spe- 

 cialized and highly complex form or group of forms. 



Such lines of progressive bodily development are 

 well marked in the backboned animals or vertebrates, 

 particularly in the mammals and the reptiles. We 

 may illustrate this point by a consideration of the 

 history of the horses. 



At present there are living on the earth about ten 

 different kinds of horses all but one of which, the 

 domestic horse, which is not found in a wild state, 

 are confined to Asia and to Africa, most of them, 

 the various kinds of zebras, living in Africa. 



But in the Pleistocene or Ice Age many different 

 kinds of horses roamed over all the continents except 

 Australia. According to Dr. James W. Gidley there 

 were a number of different kinds in North America 

 where they ranged north to beyond the Arctic circle 

 in Alaska. 



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