376 



THE CHANGING GENERATIONS 



ships among organisms that are the result of evolution. To attain this 

 end it may use any relevant data. Comparative morphology is necessarily 

 the foundation of the system, for morphological data can be obtained for 

 any species of which specimens are available. Wherever possible taxonomy 

 also incorporates the findings of comparative embryology, comparative 

 physiology, paleontology, biogeography, and genetics. 



The assignment of organisms to systematic categories is not arbitrary. 

 It is based upon shared homologies. Related species are placed together in 

 one genus because they possess many characteristics in common. The 

 species of related genera have fewer common characters, which are used 

 to define a family, and so on up the scale. The more species a category 

 includes, the fewer and more fundamental are the features common to all 



those species. To illustrate this 

 point, let us turn again to the 

 squirrels. 



The tree squirrels (Sciurus) and 

 flying squirrels (Glaucomys) share 

 with the ground squirrels (Citellus), 

 chipmunks (Tamias, Eutamias), 

 and woodchucks (Marmota) numer- 

 ous features that indicate common 

 descent. The species of these genera 

 are therefore included in a single 

 larger category, the family Sciuridae. 

 All members of the Sciuridae have a 

 single pair of upper and lower chisel- 

 shaped incisor teeth, and they lack 

 canine teeth. These characteristics 

 are also shared (along with many 

 others) by the pocket gophers 1 (family Geomyidae), pocket mice and 

 kangaroo rats (Heteromyidae), true rats and mice (Muridae), porcupines 

 (Erythizontidae), and beavers (Castoridae). The species of all these 

 families together constitute the order Rodentia, or rodents. (Note that 

 all these family names end in idae; this is the identifying mark of a family 

 name in zoology, while in botany the family ending is aceae.) Nearly all 

 rodents are small. They have rootless, chisel-like incisor teeth, a long gap 

 between incisors and molars, and no canines. 



The pikas or conies (Ochotonidae) and the hares and rabbits (Leporidae) 

 were long placed in the Rodentia, chiefly because they also have gnawing 

 teeth. There were two suborders, the Simplicidentata for the true rodents 

 and the Duplicidentata for- the pikas, hares, and rabbits. The latter were 



1 The burrowing mammals known as "salamanders" in parts of the South, including 

 Florida. 



Fig. 24.14. A beaver, Castor canadensis, 

 family Castoridae, order Rodentia. (Photo 

 courtesy Chicago Natural History Museum.) 



