THE INSTINCTS OF ANIMALS 155 



up with lightning-like rapidity when the lizard rushes at it, because 

 this rapid movement, which it sees, acts as the stimulus which liberates 

 the flight-instinct, and this works so promptly that in most cases the 

 insect is rescued from danger. Its disposition, however, is not other- 

 wise affected by its narrow escape, and it obeys anew the food-instinct 

 which impels it to settle again on the bait, until the flight-instinct is 

 again set a-going by the visual impression of the re-advance of the 

 lizard. It is the plaything of its instincts, a machine which works 

 exactly as it must. That it is only sense-impressions and not concep- 

 tions which here liberate the actions can be well seen in the case of 

 shy species of butterfly like our purple emperor (Apatura iris), which 

 flies up like lightning from the moist wood-paths on which it loves to 

 settle as soon as any rapidly moving visual image, even if it be 

 only a shadow, strikes its eyes. For this reason the collector tries to 

 approach it so as not to throw his shadow before him, for then the 

 insect lets the advancing enemy get quite close, and only flies up when 

 the net is quickly thrust towards it. In all probability the eye of 

 this insect is particularly well adapted for perceiving movements, and 

 certainly the flight-instinct reacts very promptly to such visual im- 

 pressions, and we can understand that it must have been so regulated 

 if, as we assume, the regulation came about through processes of 

 selection, for the enemies of the butterflies, such as birds, dragon-flies, 

 and lizards shoot quickly out on their prey, and therefore those 

 butterflies must always have survived whose instinct impelled them 

 to take to flight most quickly. 



In this, then, as in a thousand other cases, the instinct of flight, 

 or indeed any other mode of movement, cannot be interpreted as an 

 ' inherited habit/ because there is no evidence of the possession of 

 that degree of intelligence which could have induced the variation in 

 the previous habit, that is, in manner of movement. The same is true 

 of animals of low intelligence in regard to all the other instincts, 

 which otherwise might seem to be explicable in terms of the 

 Lamarck ian principle. 



In addition, there is a whole large group of instincts in regard 

 to which the idea of the Lamarckian principle cannot be entertained, 

 as I showed years ago, and it consists of all those instincts which are 

 only exercised once in the course of a lifetime. These cannot 

 possibly depend on practice in an individual lifetime, and trans- 

 mission of the results of this exercise to the following generation ; 

 they can therefore only be interpreted in terms of selection, unless 

 we are to give up all attempts at a scientific interpretation, and simply 

 accept them as ' marvels.' 



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