XV. EXERCISE ON CLASSIFICATION 



The science of classification or taxonomy is that branch of biology which 

 attempts to discover the natural relationships of organisms, and to put all known 

 forms of life in their proper places in a genealogical tree. Classification is based 

 entirely upon anatomy, both adult and embryonic, and is not in the least con- 

 cerned with function. 



Classification starts with the conception of the Species, which may be defined 

 for our purpose as a group of organisms essentially alike in all the details of their 

 structure. Thus all of the frogs given out in the laboratory are so nearly alike 

 that the laboratory instructions even down to the microscopic structure apply 

 to all individuals. Such frogs therefore constitute a species and are given a 

 name, called the specific name, which is in this case pipiens. There are many 

 other kinds of frogs which are quite similar to this one but differ in small details ; 

 for instance, the bullfrog is larger, has a different color pattern, and lacks the 

 dermal plicae; it therefore receives another specific name, catesbiana (see Holmes, 

 pp. 18-21). There are in fact about one hundred and forty different kinds of 

 frogs, each of which has a specific name. In order to express the fact that all of 

 these frogs are very similar to each other they are placed together into one group, 

 called a genus. This genus to which the frogs belong is called Rana, from the 

 Latin word meaning "frog," and this name is spoken of as the generic name. The 

 full name of our common frog is therefore Rana pipiens. This system of naming 

 animals with two names is called the binomial system of nomenclature and was 

 devised by Linnaeus, one of the early biologists who became interested in classi- 

 fication (see Hegner, p. 270). 



Among the members of a species there are often minor variations, which are 

 distinguished as varieties, when sufficiently important and common. Varieties 

 occur most frequently in domestic animals. See, for instance, Hegner's Fig. 160 

 (p. 294) for a photograph of the varieties of the domestic pigeon. 



All of those genera which are naturally related to each other as shown by 

 their structure are united together into a family. Thus the lions, tigers, leopards, 

 lynxes, and other catlike animals form a natural family, the Felidae, or cats, 

 having sharp fangs, and retractile claws. Similarly the dogs, wolves, foxes, etc., 

 form another natural family, the Canidae, in which the claws are not retractile; 

 the bears are another family; so also the hyenas, and the otters, weasels, and 

 martens. All of these families, together with many others which have not been 

 mentioned, have certain characters in common, such as that they all are carnivo- 

 rous and have strong fangs and sharp cutting teeth, that they have claws, and 



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