DIFFICULTIES OF INDUCTIVE VERIFICATION. 433 



trasts of climate : here great expenditure for the maintenance 

 of temperature is needed, and there very little; in one 

 zone an organism is supplied with abundant light all the 

 year round, and in another only for a few months; this 

 region yields an almost unfailing supply of water, while that 

 entails the exertion of travelling many miles every night for a 

 draught. 



Permanent differences in the natures and distributions of 

 aliment greatly interfere with the comparisons. The Swal- 

 low goes through more exertion than the Sparrow in securing 

 a given weight of food; but then their foods are dissimilar 

 in nutritive qualities. There is a want of parallelism between 

 the circumstances of those herbivores which live where the 

 plains are annually covered for a time with rich herbage, but 

 afterwards become parched up, and of those inhabiting more 

 temperate regions. Insects whose larvae feed on an abundant 

 plant, as do several of the genus Vanessa on the Nettle, have 

 practically an environment very unlike that of insects such 

 as Deilephila Euphorbia, whose larvae feed on a comparatively 

 rare plant the Sea-Spurge. 



Again, comparisons between creatures otherwise akin in 

 their constitutions and circumstances, are hindered by ine- 

 qualities in their relations to enemies. Two animals, of 

 which one is predatory and has no foes but parasites while 

 the other is much pursued, cannot properly be contrasted 

 with a view to determining the influence of size or com- 

 plexity. 



Without multiplying instances, it will be clear enough 

 then that the aggregate of destructive actions, positive and 

 negative, which each species has to contend with, is so 

 undennable in the amounts and kinds of its components, 

 that nothing beyond a vague idea of its relative total can be 

 formed. 



331. Besides these immense variations in the outer 

 actions to be counter-balanced, there are immense variations 

 74 



