I.] THE DURATION OF LIFE. 1 7 



of those animals which, even in their mature state, are very 

 hable to be destroyed by others which are dependent upon 

 them for food ; but they are at the same time among the most 

 fertile of animals, and often produce an astonishing number of 

 eggs in a very short time. And no better arrangement for the 

 maintenance of the species under such circumstances can be 

 imagined than that supplied by diminishing the duration of life, 

 and simultaneously increasing the rapidity of reproduction. 



This general tendency is developed to very different degrees 

 according to conditions peculiar to each species. The shorten- 

 ing of the period of reproduction, and the duration of life to the 

 greatest extent which is possible, depends upon a number of 

 co-operating circumstances, which it is impossible to enumerate 

 completely. Even the manner in which the eggs are laid ma}'- 

 have an important effect. If the larva of the ma3^-fly lived 

 upon some rare and widely distributed food-plant instead of at 

 the bottom of streams, the imagos would be compelled to live 

 longer, for they would be obliged — like many moths and butter- 

 flies—to lay their eggs singly or in small clusters, over a large 

 area. This would require both time and strength, and the}' 

 could not retain the rudimentary mouth which they now 

 possess, for they would have to feed in order to acquire 

 sufficient strength for long flights; and — whether they were 

 carnivorous like dragon-flies, or honey- eating like butterflies — 

 their feeding would itself cause a further expenditure of both 

 time and strength, which would necessitate a still further in- 

 crease in the duration of life. And as a matter of fact we find 

 that dragon-flies and swift-flying hawk-moths often live for six 

 or eight weeks and sometimes longer. 



We must also remember that in many species the eggs are 

 not mature immediately after the close of the pupal stage, but 

 that they only gradually ripen during the life of the imago, and 

 frequently, as in many beetles and butterflies, do not ripen 

 simultaneously, but only a certain number at a time. This 

 depends, first, upon the amount of reserve nutriment accu- 

 mulated in the body of the insect during larval life ; secondly, 

 upon various but entirely different circumstances, such as the 

 power of flight. Insects which fly swiftly and are continually 

 on the wing, like hawk-moths and dragon-flies, cannot be 

 burdened with a very large number of ripe eggs. In these 



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