302 EVOLUTION AND ANIMAL LIFE 



between amphibians and fishes. A study of these ancient 

 forms also throws light on many conditions of structure in 

 modern animals, otherwise difficult to understand. An exam- 

 ple of this sort is found in the splint bones of the modern 

 horse (see Fig. 179). 



It is a fact unquestionable that a species will change on its 

 own grounds little by little with the lapse of time and the slow 

 alteration of conditions of selection. Nations change, languages 

 change, customs change, nothing is secure against the tooth of 



FIG. 178. Ancient bird with jointed tail, claws on wings, and teeth in jaws, Archcrop- 

 teryx lithographica, from the Jurassic rocks of Bavaria. (After Nicholson from 

 Owen.) 



time. This is in general true, because with time, alteration of 

 environment takes place, events happen, there is an alteration 

 of the stress of life and with this alteration all life may be acted 

 upon. 



That time-mutations in all forms of life do take place is 

 beyond question, and some have regarded these slow changes 

 as the chief agency in the formation of species. But the 

 current of life does not flow in straight lines nor in an even 

 current. Species are torn apart by obstacles, as streams are 

 divided by rocks, and the rapidity of their formation is pro- 

 portioned to the size of the obstacle and the alternations it 

 produces in the flow of life. 



We have some basis for the estimate of the duration of a 

 species. When the great glacial Lake Bonneville occupied 



