20 
MUSEUM BULLETIN NO. 13. 
This gives the food species opportunity to increase once more 
to abnormal numbers and the process begins over again in a 
diminishing degree. Thus an oscillation is started that may 
take considerable time to subside to stable normality. 
The final population of a species then, depends upon a 
complexity of influences and is the resultant of many forces 
acting along different lines and at various angles, each inter- 
related with the other and having differing values as their op- 
posing forces are readjusted. 
There is a certain definite maximum of population beyond 
which a species cannot go. We have many evidences of this. 
One of the most obvious was the case of the Bluebird in the 
winter of 1894-5 when this common species was almost w T iped 
out in the south. For five years the species increased rapidly 
to normal population and then stopped short. As far as we can 
see no new factor was introduced, no enemy absent during the 
five years of growth was present in the sixth year, the food 
supply and birth rate seemed constant throughout, but the 
increase was definitely and positively checked during the fifth 
year. It is obvious that a limit to the Bluebird population had 
been reached. 
In estimating the effects of any set of conditions upon the 
lives of a species, there will always be found one or more factors 
having a predominating influence upon it. There is always 
one critical time or stage in its life that practically determines 
its numbers. Just what this is, is difficult of determination in 
any given species. Of course, should this factor be removed 
the next most important one takes its place as the critical moment, 
and after it another appears. Any relief to be effective must 
be applied at these critical moments in their proper succession. 
For instance an increase in food supply in summer or reduction 
of enemies would not permanently profit a species if the limiting 
factor to its numbers was the sufficiency of food in winter or 
southern ranges. With the above principle in view, let us 
consider the effects of cormorants and other birds of prey upon 
salmon. 
The number of adult salmon is the result of birth rate, 
modified by the amount of food supply, less the number taken 
