Tropisms and Instincts as Adaptations 407 



There has been some discussion as to whether birds build 

 their nests in imitation of the nest in which they were reared, 

 or whether they do so independently of any such experience. 

 There can be no doubt, however, that in some birds neither 

 memory nor imitation can play any important part in the 

 result, and that they build their nests as instinctively as spi- 

 ders make webs. 



These instincts of spiders, bees, and birds appear to be 

 more complex than the reflexes and tropisms that were first 

 described. Whether they are really so, or only combina- 

 tions of simple responses, we do not yet know. That they 

 have come suddenly into existence as we now find them 

 does not seem probable, but this does not mean that they 

 must have been slowly acquired as the result of selection. 

 The mutation theory also assumes that the steps of advance 

 may have been small. 



Our account may be concluded with the recital of some 

 instincts, chosen almost at random, that serve to show some 

 other adaptations which are the result of these inborn 

 responses. 



It is known that ants travel long distances from their nests, 

 and yet return with unerring accuracy. It has been shown 

 that they are able to do this through a marvellous sense of 

 smell. The track left by the ant, as it leaves the nest, serves 

 as a trail in returning to the starting-point. Moreover, it 

 appears that the ant can pick out her own trail, even when 

 it has been crossed by that of other ants. This means that 

 she can distinguish the odor of her own trail from that of 

 other members of the colony. The sense-organs by means 

 of which the odor is detected lie in the antennae. This fact 

 accounts for certain actions of ants that have been described 

 as showing that they have an affection for each other. Two 

 ants, meeting, pat each other with their antennae. In this 

 way they are quickly able to distinguish members of their 

 own nest from those of other nests. If they are of the same 



