BEAVER DAMS. 377 



lance and care to avert the consequences. And accordingly 

 it is found that * 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 water 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 to me 

 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 



