48 IOWA ACADEMY OF SCIENCES. 



form is evolved into a more highly organized or special- 

 ized one. In the course of time the species, possibly the 

 genus, has become entirely replaced by the more modern 

 type, but to say that the earlier forms have become extinct 

 is merely to recognize their gradual transmutation into the 

 later form. If representatives of the older type persist and 

 are finally pushed to the wall by the newer one we must 

 still recognize that there is a chain in the direct line of 

 descent for the newer type in which extinction is not the 

 proper or at least the most significant term. Eohippus, 

 Orohippus, Mesohippus, Protohippus must be looked upon 

 as links in an ancestral chain individually extinct but 

 represented in the modern horse; that is, a persistent 

 type. The Ammonites on the other hand furnish an 

 example of an extinct type. "Extinction" in the one case 

 is certainly a very different thing from what it is in the 

 other. The former is evolution not extermination or 

 elimination of the type. 



Direct evolution is perhaps the prime factor in the 

 dropping behind of particular forms of animal life, extinc- 

 ion as we have been accustomed to call it. 



Of factors causing total elimination of a form or type 

 of life we may note first; changes in the physical environ- 

 ment as these are perhaps the more certain and wide- 

 spread in effect. It should be noted of course that certain 

 forms may respond to such changes and by rapid evolu- 

 tion adapt themselves to the change when they would fall 

 into the preceding category, but there have been un- 

 doubted cases where over an extensive area the changes 

 have been so radical and rapid as to obliterate a certain 

 kind of fauna. 



For example the obliteration of cretaceous seas of cen- 

 tral North America, the present plains region of the west, 

 was accompanied by the extinction of a host of marine 

 forms which seem neither to have escaped to other parts 

 of the ocean or to have evolved into any other form fitted 

 for terrestrial or fresh water existence. Striking among 

 these are the Baculites, Ammonites and other Tetrabranch 

 Cephalopods; also a large contingent of marine saurians. 



