156 INSECTS AND HEREDITY 



It is all the difference in fact between success and failure, 

 between life and death. Comparatively rarely are the 

 conditions of the struggle such as to admit of partial 

 failure or of improvement by experience. 



One special reason for the passive means of defence 

 adopted by the vast majority of insects is to be found in 

 the peculiar dangers of their structure. Especially is this 

 true of larvae, with their haemolymph contained in freely 

 communicating cavities, and subject to the pressure of 

 muscular body-walls. Hence an insignificant injury may 

 often cause death or imperfect development from the 

 quantity of fluid which is lost. 'It is, I believe, in con- 

 sequence of these facts that the various means of pro- 

 tection in larvae are almost always of a passive kind. . . . 

 Nearly all the means of defence against . . . enemies 

 [other than ichneumons, &c] are such as tend to prevent 

 the larva from being seen or touched, rarely such as to 

 be of any avail when actually attacked. There may be 

 various changes in the mode of defence, but the object 

 is always the same, — to leave the larva untouched, a 

 touch being practically fatal.' ' 



Let us consider for a moment the mental operations 

 involved in the act of profiting by experience. Consider, 

 for instance, Mr. A. H. Hamm's interesting observation 

 — since abundantly confirmed by the testimony of many 

 naturalists— that the vast majority of the individuals of 

 Hybcrnia leucophaearia (one of the common ' Winter 

 Moths') rest with the body horizontal, thus bringing the 

 lines of the wings into parallelism with the dark shadows 

 in the vertical cracks of the oak-bark.- An individual 

 which adopted a different attitude and rested so as to 

 cause the main lines of its pattern to cut the main lines 

 of its environment might indeed escape by flight ; but 

 can any one really believe that a moth, or any of the 

 ancestors of moths, could associate the special disturbance 

 and danger to which it had been exposed with the special 

 attitude it had assumed, and would as a result of that 

 association begin to make changes in its attitude ? It is 



1 Trans. Ent. Soc, Lond., 1885, pp. 321-3. 



2 Proc. Enl. Soc. Lond. for March 19, 1902, p. xv. 



