122 FROM FISH TO PHILOSOPHER 



In the early Permian, this nascent amniote stock— per- 

 haps all of fifty million years of age— separated into sev- 

 eral branches of which one was to lead to the great 

 Jurassic and Cretaceous reptiles and, through a side 

 branch, to the birds; while another branch was to lead 

 by way of the Permian Theriodontia {therion = beast; 

 odon = tooth) and the Triassic Cynodontia (kyon = 

 dog; odon = tooth), to the mammals. Here, for the mo- 

 ment, we follow the evolution of the reptiles and the 

 birds. (Figure 9.) 



The oft-quoted statement that the birds are only glori- 

 fied reptiles that have gained wings and lost their teeth 

 sums up the many basic aflBnities between these two 

 classes. The birds and mammals, on the other hand, are 

 together distinguished from all other vertebrates by their 

 warm-blooded or homeothermic (homoios = like or 

 similar; therme = heat) state, the body temperature be- 

 ing maintained by metabolic and circulatory adjustments 

 in the range of 97° to 103° F.; while the reptiles (like 

 the Amphibia and fishes) are cold-blooded or poikilo- 

 thermic (poMo5 = many-colored; therme = he3.t) , the 

 body temperature being without significant control. At 

 first sight it would seem that so notable a feature as 

 warm-bloodedness must connect the birds and mammals 

 in a common origin, but the paleontological and physio- 

 logical evidence is overwhelmingly against this supposi- 

 tion; rather it must be accepted that warm-bloodedness 

 has been evolved independently in the two classes: their 

 ancestral lines had separated by the Permian (if not ear- 

 lier) and at a time when the nascent amniote was still 

 cold-blooded. 



The birds (or a bird) first appear in the fossil record 

 of the Upper Jurassic, in the fine-grained lithographic 

 sandstone (such as that from which Currier and Ives 

 and other nineteenth-century hthographers made their 

 prints) mined in the quarries at Solenhofen, Bavaria. 

 Only two such fossil birds (and one feather) are known, 

 and though they closely resemble each other they are 



