162 EVOLUTION AND GENETICS 



of their habitat would be reflected in their distribution at 

 once ..." 



So definite is the geological record of vertebrate evolution that 

 these changes of fauna, correlated with the varying climates of the 

 world, tell us of the influences which were active in shaping the 

 group to which we belong. To these influences we owe not only 

 the present state of the fauna of the world, but even our own ex- 

 istence. 



Step by step, through the successive conditions imposed upon 

 the phylum by swift waters, increasing aridity, the drying up of 

 streams and bodies of water, and finally changes of climate we have 

 come from these remote invertel^rate ancestors to our present 

 state (Fig. 89), not suddenly, but through a wonderful inherent 

 power to meet varying conditions successfully. By such easy 

 steps have the vertebrate classes originated. Each in itself has 

 made some adjustment to the various possibilities of the modern 

 world, but only in the whole do we find the maximum power of the 

 phylum as it is thus expressed. Upon future possil^ilities we can 

 only speculate. 



Summary. The evolution of the vertebrates is characterized 

 by the completeness of all kinds of evidence and is therefore well 

 estabUshed. The origin of the phylum is, however, obscure. It is 

 explained by several theories, among which the annelid and A7n- 

 phioxus theories combine to suggest a logical explanation of the 

 transition from invertebrate to vertebrate. Within the phylum 

 the origin of the terrestrial classes from their primitive aquatic 

 ancestors is a major step, but the existence of modern fishes with 

 some adaptations for l^oth hal)itats is proof of the possibility of 

 such a transition. The most primitive terrestrial forms were 

 Amphibia. Conditions of increasing aridity favored the develop- 

 ment of the wholly terrestrial reptiles which later became highly 

 diversified. The birds and mammals probably originated from 

 different groups of reptiles and independently acquired the im- 

 portant vaso-motor apparatus and circulatory system in which 

 they are so similar. Development of the birds, however, has been 

 concentrated upon the perfecting of flight while development of 

 the mammals has resulted in structural diversity involving a 

 considerable degree of evolution. 



