376 ANIMAL INTELLIGENCE. 



It is truly an astonishing fact that animals should 

 engage in such vast architectural labours with what 

 appears to be the deliberate purpose of securing, by 

 such very artificial means, the special benefits that arise 

 from their high engineering skill. So astonishing, indeed, 

 does this fact appear, that as sober-minded interpreters of 

 fact we would fain look for some explanation which would 

 not necessitate the inference that these actions are due to 

 any intelligent appreciation, either of the benefits that 

 arise from the labour, or of the hydrostatic principles 

 to which this labour so clearly refers. Yet the more 

 closely we look into the subject, the more impossible 

 do we find it to account for the facts by any such easy 

 method. Thus it seems perfectly certain that the bea- 

 vers, properly and strictly speaking, understand the use 

 of their dams in maintaining a certain level of water. 

 For it is unquestionable that in the solid-bank dams, as 

 already observed, a regular opening or trough is cut at one 

 part of its crest to provide for the overflow ; and now it 

 has to be added that this opening is purposely widened or 

 narrowed with reference to the amount of water in the 

 stream at different times, so as to ensure the maintenance 

 of a constant level in the pond. Similarly, though by 

 difierent means, the same end is secured in the case of the 

 stick dams. For ' in most of these dams the rapidity or 

 slowness with which the surplus water is discharged is 

 undoubtedly regulated by the beavers ; otherwise the level 

 of the pond would continually vary. There must be a con- 

 stant tendency to enlarge the orifices through which the 

 water passes,' when the stream is small, and vice versa ; 

 otherwise the lodges would be either inundated or have 

 their sub-aquatic entrances exposed.^ Moreover, a very 

 little consideration is enough to show that in stick dams 

 the tendency to increased leakage from the eflfects of per- 

 colation, and to a settling down of the dam as its materials 

 decay from underneath, must demand unceasing vigi- 



' In times of considerable * freshet ' the former case sometimes, 

 occurs ; the beavers not being able to provide for a very considerable 

 overflow through their dams, the latter become then wholly submerged. 

 "When again exposed, the animals take great pains in repairing the in- 

 juries sustained. 



