236 COMPARATIVE ZOOLOGY. 



The subkingdoms are grouped into two great Series 

 (Protozoa and Metazoa), according to their hietological 

 structure and mode of development. 128 



These terms were invented by Linnaeus, except Family, 

 Subkingdom, and Series. To Linnaeus we are also in- 

 debted for a scientific method of naming animals. Thus, 

 a Dog, in Zoology, is called Canis familiaris, which is the 

 union of a generic and a specific name, corresponding to 

 the surname and the Christian name in George Washing- 

 ton, only the specific name comes last. It will be under- 

 stood that these are abstract terms, expressing simply the 

 relations of resemblance : there is no such thing as genus 

 or species. 



Classification is a process of comparison. He is the 

 best naturalist who most readily and correctly recognizes 

 likeness founded on structural characters. As it is easier 

 to detect differences than resemblances, it is much easier 

 to distinguish the class to which an animal belongs than 

 the genus, and the genus than the species. In passing 

 from species to classes, the characters of agreement be- 

 come fewer and fewer, while the distinctions are more 

 and more manifest; so that animals of the same class are 

 more like than unlike, while members of distinct classes 

 are more unlike than like. 



To illustrate the method of zoological analysis by search- 

 ing for affinities and differences, we will take an example 

 suggested by Professor Agassiz. Suppose we see together 

 a Dog, a Cat, a Bear, a Horse, a Cow, and a Deer. The 

 first feature which strikes us as common to any two of 

 them is the horn in the Cow and the Deer. But how 

 shall we associate either of the others with these? We 

 examine the teeth, and find those of the Dog, the Cat, and 

 the Bear sharp and cutting ; while those of the Cow, the 

 Deer, and the Horse have flat surfaces, adapted to grind- 

 ing and chewing, rather than to cutting and tearing. We 



