10 INTRODUCTION [CH. 



two new types are formed. Individuals belonging to the same 

 species which differ from one another in well-marked characters 

 are termed varieties, and the laws governing the inheritance of 

 character when two varieties are mated together were worked out 

 by an Augustinian monk named Mendel, whose work has been 

 repeated and extended by a flourishing school of modern investiga- 

 tors. These laws show how when two varieties of a species exist, 

 new varieties can result from their crossing, but they throw no light 

 on a question of cardinal importance, how the varieties themselves 

 came into existence in the first case. In a few cases however, the 

 sudden appearance of these new varieties has been actually recorded. 

 In all these cases the new variety may be described as a cripple ; it 

 is characterised by the loss or imperfect development of some character 

 found in the normal form. A variety is to be discriminated from a 

 race ; a race is a subdivision of a species occupying usually a definite 

 area and discriminated from neighbouring races by a multitude of 

 small characters. 



It is obvious that so vast a science as Zoology must be divided 

 into various branches, since the different questions it 

 of B zooi C h gy. seeks to solve require that special attention should 

 be given to each side of the subject. Thus, the 

 nature and conditions of the metabolism and the mechanism by 

 which movements are effected, etc., constitute the subject-matter of 

 Physiology; the investigation of the structure of individuals and 

 of the differences in structure between the various species and the 

 search for the causes of these differences is termed Morphology; 

 whilst Bionomics is the name given to the study of the means 

 whereby an animal obtains its food and orders its life, in other 

 words, of its habits. But it must be remembered that all such 

 divisions are purely arbitrary, and indeed no great progress can be 

 made in any one department if the others be ignored. Bionomics, 

 when followed to its sources, passes into Physiology, and in trying 

 to explain the different structures studied in Morphology constant 

 recourse must be had to both Physiology and Bionomics. 



Of all. divisions of the subject, that of Physiology has been most 

 neglected it has indeed been studied systematically only in the case 

 of man and of a few of the higher animals. Hence this work will be 

 mainly concerned with the questions of Morphology and Bionomics. 

 Of these questions, by far the greatest is the problem how the dis- 

 tinctions between the various species are to be explained. The question 

 of the "Origin of Species" involves nearly all others in Zoology. 



