VARIETIES OF DUNS. 63 



these fresh relays appear, and as the season ad- 

 vances these again assume the gradatory shades 

 of red, brown, violet, and claret colour. 



6 The daily appearance of the ephemerae, as well 

 as the colours they shine in, is 'also under the in- 

 fluence of times and seasons. Excess of cold, as 

 well as of heat, is unfavourable to them : thus in 

 the depths of winter they are not seen at all ; in 

 spring they do not show themselves until towards 

 noon ; while in the meridian splendour of the 

 summer days, they (to avoid heat) come out in 

 the mornings and evenings principally, and are 

 hardly seen at midday. A kind and provident 

 nature so suits the supply of her creatures, that 

 her economy may never be disturbed. If these 

 ephemeral flies appeared all of them at one time, 

 the air would be vitiated, and the birds and the 

 fishes which live on them would be glutted to 

 satiety and fatal repletion at one time, and at 

 another might want the necessary support. But 

 we see species after species arrive in succession to 

 fill up the breaks made by the last ; and that no 

 inconsiderable interruption may occur by variation 

 in temperature, and changes in weather, some are 

 destined to appear when cold and storms prevail, 

 while others require cloudless skies and genial 

 warmth to draw them from their seclusion. The 

 whirling dun thus frolics in the gale, the red 

 spinner dances in the sunny beam, and the blue- 



