4 NATURAL HISTORY OF AQUATIC INSECTS 



form an order or a family, but comprise a miscel- 

 laneous selection of genera and species from many 

 families and several orders. Let us consider what may 

 be the significance of this remark. A family or an order 

 of Insects is a group which the naturalist supposes 

 to have descended from a common stock. An order 

 may include many families, and the common stock of 

 the order is therefore presumed to be much more 

 remote than that of any of its component families. 

 Now if all Insects were at one time terrestrial (and of 

 this we have no sort of doubt, for reasons which will 

 shortly be given), and if we find aquatic Insects in two 

 different orders, which cannot be supposed to be 

 derived one from the other, we believe that the 

 adaptation to aquatic life must have been effected 

 independently. For the common origin of both 

 orders is a stock unknown to us by observation, but 



A 



which we have good reason to suppose was not 

 aquatic. When we have a smaller group, such as an 

 aquatic family, to consider, we must ask whether it is 

 more likely that the common stock from which it is 

 descended was aquatic or not. In a few cases there 

 is reason to believe that two or more aquatic families 

 have descended from a common stock which was 

 aquatic also, but this is unusual. The aquatic families 

 of Insects are seldom wholly aquatic, they generally 

 include some terrestrial forms, and these axe, prima facie 

 the more primitive, or less altered in the matter of 

 habitat. \Ye do find some cases of families all of which 

 are aquatic, and so related to allied families which 

 are in the same case, that it is most likely that the 

 common stock in which all originated was aquatic 



