CLASSIFICATION AND DISTRIBUTION 371 



viously mentioned among dogs are very exceptional, for as a 

 rule the members of the same species are closely alike. 



Just what biologists mean by species, and just what line they 

 would draw to separate two species from each other, cannot be 

 stated. It is quite impossible to say how unlike two animals 

 must be to constitute two species, since sometimes, as with 

 pigeons, members of the same species may be very unlike, 

 while in other cases, as with sparrows, animals belonging to 

 different species are very closely similar. It has been quite 

 common to regard all animals that can breed together and pro- 

 duce fertile offspring, as belonging to the same species. But 

 this is not an accurate definition of the term, for there are many 

 animals, so different from each other that they certainly deserve 

 to be ranked as different species, but which can breed together. 

 Nor can we get any idea as to the meaning of the term "species" 

 by studying the number of similar individuals. Some species are 

 composed of an immense number of individuals, as in the case of 

 the dandelion; while other species comprise very few animals, 

 sometimes only one or two having been found. Sometimes, too, 

 the organisms belonging to the same species show a number of 

 sub-groups, and the biologist calls them sub-species, or varieties. 

 All of these facts show that no naturalist can at the present time 

 exactly define the term "species," or state definitely how species 

 may be separated from each other. When we recognize that 

 new types are constantly arising from old ones by the process 

 of divergence, it will be seen that we could not always expect 

 to draw sharp lines separating the new and the old types that 

 have arisen from a common center. But although naturalists 

 are not able to define the term accurately, or separate the species 

 strictly from each other, species are always recognized and form 

 the starting point for classification. 



Genera. A little study shows at once that some species have 

 a much greater resemblance to each other than they do to others. 

 For example, naturalists recognize the domestic cat as constitut- 

 ing one species, and the wild cat as a second. But it is quite 



