34 THE STUDY OF ANIMAL LIFE CHAP. 



millions by the end of a week. A female aphis, often 

 producing one offspring per hour for days together, might 

 in a season be the ancestor of a progeny of atomies 

 which would weigh doAvn five hundred millions of stout 

 men. The ovary of a fish like a cod may produce 

 several millions of ova ; if all these developed into young 

 fishes, the sea would soon be solid. The unchecked 

 multiplication of a few mice or rabbits would soon leave 

 no standing-room on earth. When the prolific increase 

 of numbers passes a certain limit, there may come about 

 an extraordinarily keen struggle for existence, but in 

 most cases the web of life has been so adjusted in the 

 course of ages that the over-population is nipped in the 

 bud. 



When living creatures in face of limitations and diffi- 

 culties do in any way thrust and parry, trying to get 

 food when it is scarce, striving to keep their foothold 

 when it is slippery, endeavouring to secure their well- 

 being when it is imperilled, there in a hundred forms is 

 the struggle for existence. It does not mean merely 

 that there is great mortality, especially among young 

 creatures ; the central idea is that of " answering back ' 

 to difficulties. When no thrust or parry is possible, 

 there is no struggle for existence in the strict sense. 

 Moreover, the term is not strictly applicable to cases 

 where some effective response or device has been dis- 

 covered and established, as the indirect result of previous 

 struggle and sifting ; for when a successful answer to a 

 particular difficulty can be given equally well by all the 

 members of the species, the struggle for existence has 

 been over-passed in regard to that particular point. 



If we are to think accurately of what goes on in nature 

 we must recognise that the struggle includes much more 

 than competition to the death, that it includes a great 

 variety of answers back. Some answers mean intenser 

 competition, but some mean an experiment in co-opera- 

 tion and mutual aid. At one time the effective answer 

 is to sharpen tooth and claw ; at another time the answer 

 is to make a safer nest for the young. The issue of the 

 struggle may be the persistence or the extinction of a 



