CLASSIFICATION OF ANIMALS 543 



616. Polymorphism. — When the variation within a species is marked, 

 when the different forms are not connected by intermediate gradations, 

 and when they can be correlated with some other factor or condition, then 

 the species is called polymorphic and is said to exhibit polymorphism. 

 If there are but two types, dimorphic and dimorphism are usually the 

 terms used. Sexual dimorphism is present in the large majority of 

 animals, the male and female being recognizable by characteristic fea- 

 tures. What might be called sex-linked polymorphism is shown by some 

 butterflies, the males of which are all alike, the females being of different 

 types; in other animals the males may be polymorphic. Polymorphism 

 may also be geographical, climatic, seasonal, or social. Many widely 

 distributed animals are represented by so-called subspecies, varieties, or 

 geographical races, which are often also directly related to climatic condi- 

 tions and represent geographical and climatic polymorphism. Insects 

 which have several broods in a year may exhibit seasonal variation ; and 

 ants and bees are examples of social polymorphism, accompanied by 

 division of labor. Some tropical butterflies show several color variations 

 which may all appear together at the same time and place. 



617. Basis of Classification. — The basis of classification is, of course, 

 the resemblances and differences which exist between animal types. 

 It has been seen, however, that resemblances may be of two kinds, homol- 

 ogous and analogous. It has also been shown that resemblances may be 

 due to convergence, which, of course, means analogy, and that differences 

 may be due to degeneration and retrogression. All of these tend to 

 interfere with a recognition of natural relationships among animals. 

 The biogenetic law has a bearing here, and it is usual to consider evidence 

 from early stages in the life of an animal as very important in revealing 

 its real affinities. All of these things have to be carefully considered in 

 arriving at a truly natural classification. 



618. Basis of Nomenclature. — Our nomenclature is based upon a 

 proposal made by Linnaeus that the name of each animal shall be com- 

 posed of two parts: the name of the genus and of the species to which it 

 belongs. This is what is known as binomial nomenclature. In recent 

 years there has been a tendency, especially in connection with certain 

 groups of animals, to add to these scientific names a third, or subspecific, 

 name, and even in some cases a fourth, or varietal, name. If this prac- 

 tice becomes general, our nomenclature will become either trinomial or 

 quadrinomial. At present it remains distinctly binomial. The method 

 of citation of a species was described in Chap. XIV. 



619. Rules of Nomenclature. — It has been stated that Linnaeus 

 knew only a limited number of species. His work proved such a stimulus 

 to systematic zoology that the number of these rapidly multiplied. Con- 

 tributions were published in all parts of the world and in all civilized 

 languages, and the number of species became so great as to cause much 



