THE WONDERS OF LIFE 



carnivora and ungulata, the leaping kangaroos and 

 jerboas, the burrowing moles and hyperdaei, the flying 

 cheiroptera and bats, the fishlike swimming sirens and 

 whales, and climbing lemures and apes. In all these 

 and the remaining orders of the mammals the whole 

 regular structure of the motor apparatus is strikingly 

 adapted to the habits of life which have been formed by 

 this adaptation itself. Nevertheless, we see that the 

 essential character of the inner organization which dis- 

 tinguishes the mammals as a class is not affected by this 

 adaptation, but constantly maintained by heredity. 

 These recognized facts of comparative anatomy and 

 ontogeny, and the concordant results of paleontology, 

 prove convincingly that all living and fossil mammals, 

 from the lowest ungulates and marsupials to the ape 

 and man, have descended from one common stem-form, 

 a pro-mammal, that lived in the Triassic Period; its 

 earlier ancestors in the Permian Period were reptiles, 

 and, in the Carboniferous Period, amphibia. Among 

 the characters of the locomotive apparatus which are 

 peculiar to mammals we have, on the one hand, the 

 structure of the vertebral column and the skull, and, on 

 the other hand, the formation of the muscles which are 

 attached to these supporting organs. In the skull we 

 particularly notice the formation of the lower jaw and 

 the joint by which it is connected with the temporal 

 bone. This joint is temporal, and so distinguished from 

 the square joint of the other vertebrates. The latter is 

 found in the mammals in the tympanic cavity of the 

 middle-ear, between the hammer (the modified joint of 

 the lower jaw, articular e) and the anvil (the original 

 quadratum). In harmony with this reraarkable modifi- 

 cation of the maxillary joint, the corresponding muscles 

 have naturally also undergone a considerable trans- 

 formation. A distinctive muscle that is only found in 

 the mammals and regulates their respiration is the dia- 



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