236 
COMPARATIVE ZOOLOGY. 
The subkingdoms are grouped into two great Series 
(Protozoa and Metazoa), according to their histological 
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 familiar is , 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 
