24 FUNCTION OF PROPAGATION. 



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

 these are preserved through the winter. 



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

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

 species. Calculations have been made of the numbers which an aphis may produce in a 

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

 calculation, 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 numbers ; but it is gratifying to observe, that where the powers of increase 

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

 multiplied 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. 

 The 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 numbers 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 effectual restraint upon inordinate increase. We may conceive of what is possible; 

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

 transient, and recurs only at wide intervals. 



