( cxviii ) 



Want of food. By analogy to the Malthusian doctrine that 

 human population tends to increase faster than the means of 

 subsistence, one is apt to think of the using up of the food 

 of a species as the cause that usually prevents an inordinate 

 increase of its numbers. But this cannot be so. It seems 

 clear that the land does not in general bear the full comple- 

 ment of insects that the food it supplies is able to maintain, 

 or anything approaching to that complement,* and there may 

 be advantages in working this out from our own personal 

 experience as entomologists, in order that we may realise its 

 significance. I therefore offer some observations on the 

 subject. 



Consmn2ytion of food hy herbivorous insects. 



Considering these, in the first instance, in the mass,t we 

 meet with much food destruction of a wholesale kind. 

 Aphides, by attacking young shoots and leaves, and birds 

 and molluscs by destroying seeds and seedlings, cut off an 

 immense quantity of vegetable growth in its prime ; but with 

 all this and other wholesale destruction the amount of vegeta- 

 tion that attains full maturity and then falls to the ground, 

 without having been consvimed in supporting the life of 

 herbivorous insects, appears to be veiy great in proportion to 

 that which they consume, thus leaving a very large surplus 

 fit for their consumption, but not, in fact, consumed by them. 



Let me proceed to some familiar instances, where it is 

 plainly impossible to say that the number of individuals of 

 any species has been kept down by the exhaustion of their 

 food supply. Look at a common rough untrimmed country 

 hedge or ordinary wood. As we hunt along its borders for 

 the imago, or it may be for the larva of common leaf-eating 



* See accordingly Weismann's " Evohition Theory," English translation 

 by Mr. and Mrs. Thomson, vol. i, p. 45 et seq. 



t I have no i)ersonal acquaintance with those equatorial or tropical 

 regions where there are no marked seasons, and where growth, multiplica- 

 tion, and destruction are rapid, simultaneous, and uninterrupted. But I 

 have seen no reason to suppose that the general state of things is, in this 

 respect, other than in seasonal regions, which, with their larger area, do 

 not seem second in importance, and in which it does not appear likely 

 that tlie relation between the growing vegetation and the insects that prey 

 on it, though it may be less intense in degree, can be essentially different 

 in kind. 



