136 Biology in America 



the extinction of many of the latter, by destroying their eggs 

 with sharp gnawing teeth which well served them for this 

 purpose. 



Although the origin of mammals is uncertain we find a 

 possible source in a group of reptiles known as cynodonts 

 from the dog-like character of their teeth, which occur in 

 triassic rocks in South Africa. The skull in many respects 

 resembles that of a mammal, while in others it shows reptilian 

 characters. 



But the cynodonts are found in the Trias, at the very be- 

 ginning of the Mesozoic era, at a time when the great rep- 

 tilian tree was but a slender sapling, while the * ' age of mam- 

 mals" does not commence until the close of the Mesozoic 

 era many millions of years later. What happened then to 

 retard mammalian development during the a^ons of time in 

 which ' ' great oceans waxed and waned and tiny hills to moun- 

 tains grew " ? It is possible that during all this time our an- 

 cestors were living, like the Israelites of old, in bondage to the 

 Pharaohs, who in this instance were represented by the car- 

 nivorous reptiles; but when the "first born" of the reptiles 

 were cut down and the reptilian stock smitten by the inex- 

 orable hand of time, then the mammals arose, to take their 

 il place in the sun" and become the " lords of creation." Or 

 perchance the available food supply was not abundant at the 

 time of their birth and thus their development was checked 

 until a more favorable season. 



"Perhaps the most remarkable thing which the history 

 of the Mesozoic brings forth is the immense period of evo- 

 lutionary stagnation on the part of the mammals. They are 

 first actually recorded in the Upper Triassic rocks of three 

 rather remote localities, North Carolina, Germany, and South 

 Africa, and are already differentiated in dietary habits. 

 During the Mesozoic, they develop in numbers and to a cer- 

 tain extent in tooth specialization. They do not, however, 

 increase markedly in size, but are humble folk, so far as our 

 records have revealed them, until the extinction of the dino- 

 saurs has been accomplished. One cannot but associate the 

 idea of mammalian suppression with that of dinosaurian 

 dominance in the relation of cause and effect, unless it shall 

 some day be revealed that the mammals were undergoing a 

 marked evolution beyond the temperature-limited habitat 

 of the reptiles. That the former showed no marked evolu- 

 tionary advance in the place where the dinosaurs actually 

 occurred is an attested fact, and the significance of the dino- 

 saurian check is no more graphically shown than by two 

 specimens in the Yale Museum. . . . The figure here repro- 

 duced is from a simultaneous photograph of these two speci- 



