THE PHYLA HEMICHORDATA AND CHORDATA 



insulating covering for the body. The birds represent an advanced type of 

 vertebrate animal which might have become dominant upon land instead of 

 the mammals; they might even have evolved a level of intelligence com- 

 parable with that of man. Instead, the integrative mechanisms of birds, like 

 those of insects, evolved with emphasis on instinctive, inherited behavior 

 patterns, apparent, for example, in their nesting habits, migratory flights, and 

 other activities. 



A review of existing birds is impossible within the limits of this chapter. 

 Despite diversity in appearance and habits, they are a homogeneous group 

 when compared with such classes as the reptiles, living and extinct, and the 

 mammals. The flightless birds (Palaeognathae) seem to have originated from 

 flying ancestors, rather than from ancestors in which the power of flight had 

 never been developed. Loss of flight in these birds may have been related to 

 their isolation in such regions as Australia, New Zealand, and Madagascar, 

 where there were few if any carnivorous enemies. Apparently the ability to 

 fly has been lost in a number of independent lines of birds during the evolu- 

 tion of the class. Many recent birds have become modified in relation to 

 aquatic life, although none so completely as the penguins (Impennes). Most 

 characteristic of existing bird life are the small perching birds familiar 

 as our common songsters. 



The Mammalia. A mammal might be loosely defined as a reptile further 

 specialized for terrestrial life. Mammals can be defined technically as 

 vertebrates with hair and mammary glands, without specifying the less 

 obvious characteristics by which they may be further distinguished. In 

 intelligence they are the highest class of vertebrates, and we think of 

 them as most advanced in complexity of structure, although birds and mam- 

 mals represent comparable climaxes in vertebrate specialization. The 

 characteristics by which mammals are better qualified than reptiles for 

 terrestrial life are their temperature regulation, their manner of reproduction, 

 their mechanisms of locomotion, and, by no means least, their brains. To a 

 lesser extent their eyes, ears, and teeth seem more efficient than those of 

 reptiles. The comparison presents the mammal as an animal more capable 

 of going into action, maintaining action, and reproducing its kind than any 

 of its reptilian ancestors or contemporaries. 



The class Mammalia is subdivided into three subclasses, on the basis of 

 clear-cut diflferences in reproductive habits. The subclass Prototheria includes 

 the oviparous, or egg-laying, mammals: Ormthorhynchus, the platypus, and 

 Echidna, the spiny anteater. The subclass Metatheria, or Marsupialia, contains 

 the marsupial or pouched mammals: bandicoots, opossums, wombats, 

 phalangers, and kangaroos. The subclass Eutheria, or Placentalia, includes the 

 viviparous mammals, represented by all the more advanced and familiar 

 forms. These are distributed among sixteen orders, of which the following 

 are representative: 



Order Insectivora: moles, shrews, and hedgehogs. 

 Order Chiroptera: bats. 



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