112 Animal Life and Intelligence. 



character was in them preponderant, and by intercrossing 

 formed a less fluctuating, useless character than their pro- 

 genitors exhibited. Still, the extensive occurrence of such 

 neutral, or useless, characteristics would be in the highest 

 degree improbable. Our ignorance often prevents us from 

 saying in what particular way a character is useful. We 

 must neither, on the one hand, demand proof that this, 

 that, or the other specific character is useful, nor, on the 

 other hand, demand negative evidence (obviously impos- 

 sible to produce) that it is without utilitarian significance ; 

 but we may fairly request those who believe in the wide 

 occurrence of useless specific characters to tell us by what 

 means these useless characters have acquired their relative 

 constancy and fixity. A suggestion on this head will be 

 found in the next chapter. 



We must now pass on to consider briefly a most im- 

 portant factor in the struggle for existence. Hitherto we 

 have regarded this struggle as uniform in intensity; we 

 must now regard it as variable, with alternations of good 

 times and hard times, and indicate the causes to which 

 such variations are due. 



With variations of climate, such as we know to occur 

 from year to year, or from decade to decade, there are 

 variations in the productiveness of the soil ; and when we 

 remember how closely interwoven are the web and woof of 

 life, we shall see that the increased or diminished produc- 

 tiveness of any area will affect for good or ill all the life 

 which that area supports. The introduction of new forms 

 of life into an area, or their preponderance at certain 

 periods owing to climatic or other conditions particularly 

 favourable to them as opposed to other forms, may alter 

 the whole balance of life in the district. We are often 

 unable to assign any reason for the sudden increase or 

 diminution of the numbers of a species ; we can only pre- 

 sume that it is the result of some favourable or unfavour- 

 able change of conditions. Thus Mr. Alexander Becker * 

 has recently drawn attention to the fact that whereas for 



* Nature, vol. xlii. p. 136. 



