STRUCTURE AND DEVELOPMENT 43 



living in the Australian faiinal region. The duck hi 11 or 

 Ornithorhy nchus is the hetter known animal, with its 

 close fur, webbed feet, and flattened ducklikf beak, 

 while its only other near relative, the Echidna, is 

 somewhat similar to the spiny hedgehog in external 

 appearance. A unique peculiarity of these two forms 

 is that they produce eggs much like those of rei)tiles 

 and birds, and this fact, together with others of a 

 structural nature, brings the whole grou]) of maninials 

 near to the lower classes of the Vertebrata. 



Looking back on the several orders of mammals, it 

 will be seen that the last mentioned are much Ic.-s 

 differentiated or specialized in their general organiza- 

 tion. Above the level of the egg-layers and the pouched 

 mammals, the higher orders branch out in different 

 directions and reach up to various levels of the scale 

 of animal organization. 



The foregoing structural evidences of organic trans- 

 formation in the past histories of cats and seals and 

 whales insistently recall the analogies of the locomotive 

 and the ship employed at the outset. All these animals, 

 like the mechanical examples, have come to differ in 

 their derivation from the same original parents, and their 

 lines of descent have diverged so as to fit the products 

 of evolutionary modification to diverse circumstances. 

 Even the vestigial organs of animals have their counter- 

 parts in the machines. The cowcatcher was a large 

 and important structure in the early days of railroad- 

 ing, but it has become relatively useless with the 

 decrease of grade crossings and the construction of 

 more complete lines of fence. The structure still 

 persists, sometimes in a greatly reduced form. Even 



