Systematic Zoology 129 



commoa stock. We have seen already much to con- 

 firm this belief so far as concerns insects : that each 

 individual insect passes through a process of growth 

 from very simple beginnings, that the same structure 

 or organ can be variously modified in different insects 

 to perform different functions. The study of insect 

 classification will show us that a gradual increase in 

 the amount of difference between insects can be 

 traced. The subject is vast ; it has been calculated 

 that there are more different kinds of insects than 

 of all other animals put together. A quarter of a 

 million have already been described, and it is thought 

 that two millions more yet remain to be discovered ! 

 It seems desirable therefore to illustrate the question 

 by means of a few concrete examples. 



Variety. — On sunny hillsides in the south of 

 England, a little Butterfly, wings dark brown above 

 with a border of orange spots, can be commonly seen 

 flitting about during the month of June and again 

 during August. It is known to naturalists as Polyom- 

 matus astrarche (fig. 82). A more careful examination 

 shows in the centre of each forewing a black spot. 

 As the insect rests awhile on some plant and holds 

 its wings raised together over its back, we have an 

 opportunity of observing their under side (fig. 82 F). 

 The ground colour is much paler than on the upper 

 surface — a light greyish fawn. Around the margins 

 of all the wings, inside a series of white lunules and 

 black specks, are rows of orange spots, as on the 

 upper side. Internal to these beneath each forewing 

 comes a row of five black spots surrounded by white 

 rings ; within these again at the centre of the wing 

 a single black, white-ringed spot. The under side 

 of the hind wing has a number of similar "eye-spots" 

 scattered over its surface, and a triangular whitish 

 patch towards the centre of the space just within the 



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