BEAVER— DAMS. 
377 
lance and care to avert the consequences. And accordingly 
it is found that 6 in the fall of the year a new supply of 
materials is placed upon the lower face of these dams to 
compensate this waste from decay.’ 
Now, it is obvious that we have here presented a con- 
tinual variation of conditions, imposed by continual varia- 
tions in the amount of w T ater coming down ; and it is a 
matter of observation that these variations are met by the 
beavers in the only way that they can be met — namely, by 
regulating the amount of flow taking place through the 
dams. It will therefore be seen that we have here to con- 
sider a totally different case from that of the operation of 
pure instinct, however wonderful such operation may be. 
For the adaptations of pure instinct only have reference 
to conditions that are unchanging ; so that if in this case 
we suppose pure instinct to account for all the facts, we 
must greatly modify our ideas of what pure instinct is 
taken to mean. Thus we must suppose that when the 
beavers find the level of their ponds rising or falling, 
the discomfort which they experience acts as a stimulus to 
cause them, without intelligent purpose, either to widen 
or to narrow the orifices in their dams as the case may be. 
And not only so, but the conditions of stimulation and 
response must be so nicely balanced that the animals 
widen or narrow these orifices with a more or less precise 
quantitative reference to the degree of discomfort, actual 
or prospective, which they experience. Now it seems tome 
that even thus far it is an extremely difficult thing to be- 
lieve that the mechanism of pure or wholly unintelligent 
instinct could admit of sufficient refinement to meet so com- 
plex a case of compensating adaptation ; and, as we shall 
immediately see, this difficulty increases still more as we 
contemplate additional facts relating to these structures. 
Thus it sometimes happens that in large dams the 
pressure of the water which they keep back is so consider- 
able that their stability is endangered. In such cases it 
has been observed by Mr. Morgan that, at a short distance 
beneath the main dam, another and lower dam is thrown 
across the stream, with the result of forming a shallow 
pond between the two. This pond is — 
