Mammalia: Classification 



19 



In an earlier chapter il was pointed out (p. 381) that some charac- 

 teristics of animals undergo adaptive evolutionary change much more 

 readily than do others. For recognition of the more remote genetic 

 relationships, chief reliance must be placed on "conservative" charac- 

 teristics. Methods of reproduction are conservative. Most terrestrial 

 amphibians, even to their disadvantage, retain the reproductive 

 method of their aquatic ancestors. Aquatic reptiles are anatomically 

 well adapted to aquatic living, but their eggs require a land-and-air 

 environment. Judging modern mammals on the basis of reproduction, 

 the Class at once cleaves sharply into three divisions, usually ranked 

 as subclasses. The general comparative anatomy of mammals justifies, 

 so far as could be expected, this tripartite division on the basis of 

 reproduction. 



In the great majority of mammals, placentalia, the minute eggs 

 undergo prolonged intrauterine development, the physiologic neces- 

 sities of embryo and fetus being provided by the placenta. In a much 

 smaller division of mammals, marsupialia, the eggs, usually some- 

 what larger than those of placental mammals, develop for a relatively 

 short time in the uterus and ordinarily without formation of a placenta 

 (in a few cases a very weak placenta occurs: see p. 294), and, after 

 birth, the fetus is retained for a long period in an abdominal integu- 

 mentary pouch, the marsupium, within which are the mammary 

 glands. Two existing genera of mammals are not included in either of 

 these divisions. These are Ornithorhynchus and Echidna (mono- 

 tremata), whose reproduction is essentially reptilian. Large eggs, 

 containing much >olk and enclosed in shells, are "laid" and incubated 

 externally. After hatching, the young obtain milk produced by mam- 

 mary glands of primitive structure and devoid of nipples. This feeding 

 after birth is the only mammalian feature of reproduction in these 

 animals. 



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