12 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



species ; we believe, on the contrary, that one form has given rise to 

 another. At the same time, the common characteristic on the strength 

 of which we deem it warrantable to give a name to a group of individuals 

 must not be markedly fluctuating. The specific character should exhibit 

 a certain degree of constancy from one generation to another. 



2. Sometimes a minute character, such as the shape of a tooth or 

 the marking of a scale, is so constantly characteristic of a group of 

 individuals that it may be safely used as the index of more important 

 characters. On the other hand, the distinction between one species and 

 another should always be greater than any difference between the members 

 of a family (using the word family here to mean the progeny of a pair). 

 For no one would divide mankind into species according to the colour 

 of eyes or hair, as this would lead to the absurd conclusion that two 

 brothers belonged to different species. Thus it is often doubly un- 

 satisfactory when a species is established on the strength of a single 

 specimen, (a) because the constancy of the specific character is undeter- 

 mined ; (b] because the variations within the limits of the family have 

 not been observed. Indeed, it has happened that one species has been 

 made out of a male and another out of its mate. But the characters 

 of a single specimen are sometimes so distinctive that the zoologist is 

 safe in making it the type of a new species, or even of a new genus. 



3. While cases are known where members of different species have 

 paired and brought forth fertile hybrids, this is not common. The 

 members of a species are fertile inter se, but not usually with members of 

 other species. In fact, the distinctness of species has largely depended 

 on a restriction of the range of fertility. 



To sum up, a species is but a relative conception, convenient when 

 we wish to include under one title all the members of a group of 

 individuals who resemble one another in certain characters. There is 

 no absolute constancy in these specific characters, and one species often 

 melts into another with which it is connected by intermediate varieties. 

 At the same time, the characters, on account of which the naturalist 

 gives a specific name to a group of individuals, should be greater than 

 those which distinguish the members of any one family, should show 

 a relative constancy from generation to generation, and should be 

 associated with reproductive peculiarities which tend to restrict the 

 range of mutual fertility to the members of the proposed species. 



It will be enough now simply to state some of the more important 

 grades of classification : 



Individuals. 



Varieties among these individuals. . 



Species, e.g., Felis tigris. 



Genus, Felis. 



Family, Felidae. 



Order, Carnivora. 



Class, Mammalia. 



Phylum or Series, Vertebrata. 



[TABLE. 



