GENERAL ZOOLOGY 



phylum are few in number and superficially unlike their numerous vertebrate 

 relatives. Yet they have the gill slits, notochord, and nervous system of the 

 chordate. Since vertebrates appear as fishes in the Ordovician, it is con- 

 cluded that these first vertebrates of the fossil record must have been pre- 

 ceded by fairly complex ancestors in the Cambrian, in which representatives 

 of all the other major phyla are found. This means that a common ancestor 

 for all chordates would be sought in the Age of Invertebrates. No fossils 

 representing such an ancestor are available, and it is unlikely that any will 

 ever be found. The origin of vertebrates from some invertebrate source was 

 the subject of much speculation, based on the data of comparative anatomy 

 and embryology, when such theorizing was the vogue in the last decades of 

 the nineteenth century; nothing that proved convincing was ever made of the 

 matter, and it is unlikely that anything ever will be. The animal life of the 

 Cambrian is remote, but it was preceded by millions of years from which we 

 have virtually no fossils and during which the Cambrian types were evolved. 

 In studying evolutionary history within the subphylum Vertebrata, we have 

 access to the fossil record and also to "surviving fossils," such as the 

 egg-laying mammals, whose structure and development can be fully ex- 

 amined. Reconstructing the past from the data available, we conclude that 

 the earliest vertebrates were small, heavily armored, bottom-dwelling, fish- 

 like animals, living in fresh water rather than the salt water from which 

 their remote ancestors presumably came. From such early fishes came others 

 that made a beginning of air-breathing while still in fresh water, and some 

 of these invaded the ocean. Late in this Age of Fishes came the first land 

 vertebrates, the Amphibia, descended from the air-breathing fishes known as 

 lobe fins. In the Age of Amphibians, when the great coal measures were 

 being laid down, these vertebrates were the dominant forms upon the 

 marshy land surface. Reptiles arose from early amphibians and succeeded 

 them in the Age of Reptiles. Both birds and mammals appeared as early 

 oflfshoots from reptilian lines; the mammals remained small and insignificant 

 animals until the reptiles began to decline. In the Age of Mammals the 

 members of this class were able to range more widely than any of their 

 predecessors, because of their eff^ective locomotion, their warm-bloodedness, 

 and their mode of development within the parent. As the mammals declined 

 toward the end of the Tertiary, the human stock became differentiated from 

 other Primates; the present is sometimes called the Age of Man. 



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