2 INTRODUCTION TO ZOOLOGY 



note amongst individuals of the same kind, variations which 

 are not constant from generation to generation. 



Sometimes, however, these groups may still contain forms 

 with so much constant variation of structure that further sub- 

 division may be desirable, e.g. an order is very often divided 

 into sub-orders or tribes, and then these into families ; some- 

 times it is even necessary to distinguish separate varieties of 

 one species. 



This system may be more clearly understood if we take a 

 special example and attempt to classify it. Let us take, e.g., 

 the common, well-known Large Cabbage White Butterfly. We 

 know this butterfly by its size and colouring, the white wings 

 being marked in a special way with dark -brown pigment 

 (Fig. 149), and also we know its caterpillar by its special form 

 and colouring (Fig. 145). There are other very similar white 

 butterflies, e.g. the " Small White " and the '' Black-veined 

 White," but these two are obviously different " kinds " of 

 butterfly, as we say, for they differ slightly, but constantly, in 

 size and in details of marking from the " Large White," also 

 the habits and external appearance of their caterpillars are 

 different ; hence, whilst popularly we call them all " White 

 Butterflies," we distinguish them as Large White, Small 

 White, and Black-veined White. Such popular names, how- 

 ever, vary in different countries, and so it is convenient to 

 have Latin or Greek equivalents which are used by all 

 observers, irrespective of locality. Scientifically, therefore, 

 we include all these three forms in one genus, the genus 

 Pieris, and we express our sense of the differences between 

 them by giving each, a specific name in addition. The Large 

 White we call Pieris brassicae, the Small White, Pieris rapae, 

 and the Black-veined White, Pieris cratoegi. This method of 

 calling each kind or species of creature by a double name, was 

 introduced by Linnaeus, and it has proved a most convenient 

 one. 



Now there are several other genera of butterflies that 

 resemble Pieris in general form of wing, and in the general 

 habits and form of caterpillar and chrysalis, e.g. the Orange 

 Tip Butterfly {Euchloe cwrdamines. Fig. 153), and the Brimstone 

 (Gonepteryx rhamni) ; these, therefore, although too dissimilar 

 to be included in the same genus, are all grouped together in 

 one farnily, the Pieridae. Then again there are many different 



