54 



General Part. 



markings on the shoulders^ or even entirely black ; and similar varia- 

 tions are found in many other Mammals and in Birds ; male Stag- 

 beetles have, as a rule, very large mandibles, but individuals have- 

 been found with mandibles whose length is only a fraction of the 

 usual size. Foxes, again, are sometimes destitute of the hindmost, 

 molar in the lower jaw, etc. All these points in which an animal 

 differs from the usual type, are called individual variations. 

 In some species two or more forms may oocur, wMoh are in some respects 

 distinct from one another, and generally not connected by those transitional 



foi-ms usual with the individual varia- 

 tions just mentioned. This phenomenon 

 is called dimorphism, if the 

 species occur in two different forms ;. 

 polymorphism, if there are more 

 than two. Dimorphism is found, for 

 example, in the Heteroptera, in different 

 species of which, winged, as well ae wing- 

 less forms occur. This and similar cases, 

 are easily derived from the ordinary 

 individual variations, as the result of 

 great individual variation, the transi- 

 tional forms having disappeared. 

 Dimorphism and polymorphism are 

 fairly common amongst colonial animals ; 

 e.g., Oraspedota, Siphonophora, Octac- 

 tinia, Polyzoa; and here it is evidently 

 the result of the common life of the- 

 individuals composing the colony; since 

 they are in direct organic communica- 

 tion with one another, some have one special function, some another, and a, 

 natural result of this division of lahour, is a differentiation of the individuals.. 

 The dimorphism or pol3Tnorphism occurring among the Social Insects (Bees, 

 Ants, Termites) is due to similar causes. 



When a species has a wide distribution, it frequently happens that 

 the individuals living in one locality differ, as a rule, in some 

 respects, from the inhabitants of another : the individuals of each 

 region are then said to form a special variety (race, sub-species). 

 Hares, for example, which are distributed over the greater part of 

 Europe, all belong to a single species, but form three groups : a 

 south European, whose individuals are distinguished by short, loose 

 fur, long ears and rust-coloured back ; a Mid-European, with long and 

 close fur; and a North-East European (in North Russia), with a coat,, 

 longer and thicker than either of the others, and very white. These- 

 three groups are different varieties, for though they are in general 

 agreement, still they usually differ from one another in the 

 points just noticed ; but they are not different species, for each 

 comprises specimens which approach the type of one of the other 

 groups, so that they are not sharply defined. What has been said of' 

 Hares holds good also for various species of Mammalia and Aves. 

 which are distributed through Europe and North Asia. The Siberian 



Fig. 42. Dimorphism of aHetero- 

 pteran (Blissus leucopterus). A Winged 

 form with, fore and hind wings; B wing- 

 less form with reduced fore wings and no 

 hind wings. — After Eiley. 



