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 antenna. 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 antennez. 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 
