592 THE POPULAR SCIENCE MONTHLY. 



working singly is sufficient to develop some instincts. But, no doubt, 

 in the case of most instincts intelligence and natural selection have 

 gone hand in hand, or co-operated, in producing the observed results 

 — natural selection always securing and rendering permanent any ad- 

 vances which intelligence may have made. Thus, to take one case as 

 an illustration. Dr. Rae tells me that the grouse of North America 

 have the curious instinct of burrowing a tunnel just below the surface 

 of the snow. In the end of this tunnel they sleep securely, for when 

 any four-footed enemy approaches the mouth of the tunnel, the bird, in 

 order to escape, has only to fly up through the thin covering of snow. 

 Now, in this case the grouse probably began to burrow in the snow for 

 the sake of warmth, or concealment, or both ; and, if so, thus far the 

 burrowing was an act of intelligence. But the longer the tunnel the 

 better would it serve in the above-described means of escape ; there- 

 fore natural selection would tend to preserve the birds which made the 

 longest tunnels, until the utmost benefit that length of tunnel could 

 give had been attained. 



And similarly, I believe, all the host of animal instincts may be 

 fully explained by the joint operation of these two causes — intelligent 

 adjustment and survival of the fittest. For now I may draw atten- 

 tion to another fact which is of great importance, viz., that instincts 

 admit of being modified as modifying circumstances may require. In 

 other words, instincts are not rigidly fixed, but are plastic, and their 

 plasticity renders them capable of improvement or of alteration, ac- 

 cording as intelligent observation requires. The assistance which is 

 thus rendered by intelligence to natural selection must obviously be 

 very great, for under any change in the surrounding conditions of life 

 which calls for a corresponding change in the ancestral instincts of the 

 animal, natural selection is not left to wait, as it were, for the required 

 variations to arise fortuitously ; but is from the first furnished by 

 the intelligence of the animal with the particular variations which are 

 needed. 



In order to demonstrate this principle of the variation of instinct 

 under the guidance of intelligence, I may here introduce a few ex- 

 amples. 



Huber observes, "How ductile is the instinct of bees, and how 

 readily it adapts itself to the place, the circumstances, and the needs 

 of the community ! " Thus, by means of contrivances which I need 

 not here explain, he forced the bees either to cease building combs, or 

 to change their instinctive mode of building from above downward, to 

 building in the reverse direction, and also horizontally. The bees in 

 each case changed their mode of building accordingly. Again, an 

 irregular piece of comb, when placed by Huber on a smooth table, 

 tottered so much that the bumble-bees could not work on so unsteady 

 a basis. To prevent the tottering, two or three bees held the comb 

 by fixing their front-feet on the table, and their hind-feet on the comb. 



