^4 The Food Relations of the Carahidce and CoccinellidcB. 



entomologists, is largely due to a hasty generalization, based upon 

 insufficient data. Observations of the food of these beetles have 

 hitherto been left almost vrholly to chance, and have nowhere 

 been systematically pursued — from which it has resulted that we 

 know their habits only in the most conspicuous situations, and 

 have not a fair idea of the general average of their food. 

 Neither have observations of any kind been numerous enough to 

 enable us to detect clearly differences of food habit in different 

 species or genera of these families; but, with slight occasional 

 exceptions, all Carabid^e and Coccinellidae have been classed 

 together as essentially carnivorous. 



Besides insufficient observation, a tendency to reason too confi- 

 dently from structure to function is responsible for many mistaken 

 notions — a tendency particularly liable to mislead when applied 

 to the habits of animals. It is frequently assumed that the most 

 prpminent and peculiar adaptive structures are necessarily indica- 

 tive of the most important and customary habits, and that struct- 

 ures especially fitted for one function are thereby incapacitated for 

 every other. 



The first of these assumptions ignores the fact that many 

 adaptive structures are acquired for the sake of the advantage 

 derived, not in ordinary, but in extraordinary circumstances. The 

 struggle for existence is one of greatly varying intensity, and the 

 really decisive moments of the conflict are often only brief and 

 occasional. The time spent in actual combat by very belligerent 

 and very powerful animals, is doubtless but a small fraction of 

 their whole lives; and yet by far the most prominent and import- 

 ant of the structural peculiarities which serve to distinguish them 

 from their more peaceful allies, may be those which enable them 

 to triumph in these occasional but critical instants. Likewise the 

 pinch of starvation must commonly be felt only at rare intervals, but 

 no structures will be more thoroughly elaborated or carefully pre- 

 served than those serving to give the animal the advantage 

 during these brief periods, since the continued existence of the 

 species depends on these no less than on those of constant use. 

 From the prominent adaptive structures we may safely infer, as a 

 general rule, what the animal will do in the stress of a life and 

 death struggle, but not necessarily what are its ordinary practices. 



The second of the above assumptions is also negatived, occa- 



