M FUNCTION OF PROPAGATION. 



i&d female in a winged state, are produced, by the congress of which ova are fertilized : 

 these are pjeserved thn>ugh the winter. 



It would seem from this, that although in the articulated class budding is resorted to to 

 tamlUpIj the number of individuals, yet it is not adequate to continue the existence of the 

 apeeiee. Calculations have been made of the niunbers which an aphis may produce in a 

 tfftgle season, provided the individuals are not destroyed by accident. According then to 

 oaloolation, based upon observed facts, the whole brood in a season from a single aphis 

 will amount to the inconceivable number of 1,000,000,000,000,000,000! From such an 

 ability to increase, we should at first thought be disposed to expect a result approaching 

 the calculated nimibers ; but it is gratifying to observe, that where the powers of increase 

 Vtfe extended and multiplied as in the case of the aphis, the destructive powers are equally 

 taultiplied and active : there is therefore but little reason to fear an excessively injurious 

 multiplication in the midst of so many enemies. The aphis is extremely feeble : the touch 

 destroys it ; the winds, rains, and cold sweep off its numbers by hundreds of thousands. 

 Hie excessive multiplication of individuals is only a means of supplying an increased 

 number of consumers : hence instead of an approach to the enormous number indicated 

 by calculation, their actual numbei-s will fall infinitely short of it. The balance of nature 

 is always preserved ; and wherever danger seems to point, there checks are provided, which 

 put an eflfectual restraint upon inordinate increase. We may conceive of what is possible; 

 bnt the actual just suffices to keep up a suitable equilibrium : at most the threatening is 

 transient, and recurs only at wide intervals. 



