THE NEW EVOLUTION 



that special sense or of those special conditions. The 

 entire organization of the type tends to become con- 

 centric about the organs which are most important 

 for its welfare. The necessary uniformity in these 

 organs seems to impose a corresponding uniformity in 

 all other features. So there results a more or less 

 fixed uniformity of structure in all the animal types 

 which are activated mainly by a single special sense 

 or are confined within a relatively narrow range of 

 special conditions. An excellent illustration of this 

 is afforded by the birds. 



Among the vertebrates the least diversified of the 

 included classes is that which includes the birds. All 

 birds exhibit a similarity in the broader features of 

 their structure which, considering their numbers and 

 the very great diversity in the minor structural details, 

 is surprising. In conformity with this, birds in their 

 later embryonic stages and in their preadult existence 

 exhibit a uniformity which is without parallel among 

 the vertebrates. 



All birds lay eggs which are enclosed within a rigid 

 and at the same time brittle calcareous shell. There 

 are among them no viviparous forms such as occur 

 among the mammals, reptiles, fishes and amphibians. 

 The eggs are always large, and are provided with 

 abundant food material. From the egg the chick 

 emerges in a well developed — sometimes in a very 

 highly developed — stage. 



In all birds except the megapodes the young are 

 assiduously tended by their parents, or by one parent, 

 until nearly or quite the full size is reached. In all 



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