32 THE STORY OF ANIMAL LIFE 



into its various kinds. But how do we know when 

 a number of animals are all of one kind ? No 

 two individual animals are ever exactly alike, any 

 more than two persons are ever exactly alike. 

 " It is a matter of common observation that no 

 two individuals of a species are ever exactly alike; 

 two tabby cats, for instance, however they may 

 resemble one another in the general characters of 

 their colour and markings, invariably present dif- 

 ferences in detail by which they can be readily 

 distinguished. Individual variations of this kind 

 are of universal occurrence " (T. J. Parker). 



Among a host of animals that present so many 

 differences, how do we determine what shall be 

 considered as belonging to one and the same 

 kind ? This is a point that nature usually settles 

 thus. If two varieties when mated produce off- 

 spring which are perfectly fertile when mated 

 again with another set of offspring similarly pro- 

 duced, then the two varieties, however differing 

 in appearance, belong to one species. If on the 

 other hand, the two belong to a different species, 

 the offspring will be what is called a mule or hy- 

 brid, and will not produce offspring if mated 

 with another mule. One of the most familiar 

 examples of a mule is the animal, commonly 

 so-called, which results from mating a horse and 

 an ass, and partakes of the characteristics of 

 both. 



Every animal receives two Latin or Latinised 

 names, the first that of the genus, the second that 

 of the species; this system of naming, often re- 

 ferred to as the " binary nomenclature," we owe 

 to the industry of Linnaeus the great Swedish 

 botanist and zoologist. Genera are groups con- 

 sisting of a number of different species which 



