Tropisjns and Instincts as Adaptations 409 



ganglion being pierced, the poison acting with almost instan- 

 taneous effect. But it may be questioned whether this is 

 really necessary, and whether the same end might not be 

 gained, although not quite so instantaneously, if the cater- 

 pillar were pierced in almost any other part of the body. Can 

 we be seriously asked to believe that this instinct has been 

 perfected by the destruction of those individuals (or of their 

 descendants) that have not pierced the caterpillar in exactly 

 the middle of a segment of the anterior ventral surface ? It 

 seems to me that the argument proves too much from the 

 selectionist's point of view. If the wasp pierced the cater- 

 pillar in the middle of its back, we should have passed over 

 the act without comment ; but since the injection is usually 

 made on the ventral side, and since we know that the nerv- 

 ous system lies in this position, it has been assumed that the 

 act is carried out in this way, in order that the poison may 

 penetrate the nervous system more quickly. Yet a fuller 

 knowledge may show that there is really no necessity for 

 such precision. 



A curious response is the so-called death-feigning instinct 

 shown by a number of animals, especially by certain insects, 

 but even by some mammals and birds. Certain insects, if 

 touched, draw in their legs, let go their hold, and fall to the 

 ground, if they happen to be on a plant. It is not unusual 

 to meet with the statement that this habit has been acquired 

 because it is useful to the insect, since it may often escape in 

 this way from an enemy. This does not appear on closer 

 examination to be always the case, and sometimes as much 

 harm as good may result, or what is more probable, neither 

 much advantage, nor disadvantage, is the outcome. This can, 

 of course, only be determined in each particular case from a 

 knowledge of the whole life of a species and of the enemies 

 that are likely to injure it. 



Hudson has recorded l a number of cases of this death- 



1 " The Naturalist in La Plata." 



