The Food of Birds. 91 



tioii is uniform, because the checks upon its increase are 

 substantially unvarying, and because these two forces bal- 

 ance each other. To set up any vibration in any one of 

 these checks, will necessarily cause a corresponding vibra- 

 tion in the number of the species limited by it. More 

 explicitly, to set up an oscillation in a predaceous or 

 parasitic species must produce a reverse oscillation in the 

 species parasitized or preyed upon. As the former in- 

 creases, the latter must diminish, and vice versa. But 

 either a marked decrease or a marked inprease of a spe- 

 cies will cause it to oscillate, unless made with extreme 

 slowness, — a slowness so extreme as to allow progressive 

 adjustments of all kinds to keep pace with it. 



Taking a predaceous beetle as an example, we see that 

 a rapid decrease of its numbers, partly relieving the spe- 

 cies which it preys upon from one of the usual checks upon 

 its multiplication, will affect an increase in those species, — 

 will thus render the food of the predatory insect more 

 abundant. This will, in turn, faciliate individual main- 

 tenance of the predatory insect and thus stimulate repro- 

 duction, initiating a forward movement, which, proceeding 

 at a geometrical ratio, must continue until the predaceous 

 species becomes too numerous for its food, or reaches other 

 limitations; when destruction of the excess produced will 

 send it back below the average line again. An oscillation 

 will thus necessarily arise which must be reproduced in the 

 food species connected with it. 



On the other hand, if the predaceous species be sudden- 

 ly increased in number by a diminished powder or stringen- 

 cy in one of its accustomed checks, the process will sim- 

 ply be reversed, but the resulting oscillation will be the 

 same. The predaceous species will increase geometrically 

 until its food supply becomes insufficient for it, then by 

 starvation and diminished reproduction it will be again 

 reduced, and so on indefinitely. Any marked disturbance 

 of a fixed adjustment between the rate of reproduction and 

 the death rate, whether it result in increase or decrease, 

 whether it affects a beneficial or an injurious species, is, 

 therefore, in itself, an immediate evil ; only to be incurred 



