The Food of Birds. 
91 
Now a species is stable because the rate of its reproduc- 
tion is uniform, because the checks upon its increase are 
substantially unvarying, and because these two forces 
balance each other. To set up any vibration in any one of 
these checks, will necessarily cause a corresponding 
vibration 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 increase 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 in- 
sect more abundant. This will, in turn, faciliate individ- 
ual maintenance of the predatory insect and thus stimu- 
late reproduction, 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 power or strin- 
gency in one of its accustomed checks, the process will 
simply be reversed, but the resulting oscillation will be 
the same. The predaceous species will increase geometri- 
cally 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 dis- 
turbance of a fixed adjustment between the rate of repro- 
duction 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 
