II.] ON HEREDITY. 95 



How can practice have had any influence upon the origin of 

 the instinct which leads one of our \i\i\.\.^r'^\Q:s— {Vanessa levana\ 

 —to lay its green eggs in single file, as columns, which project 

 freely from the stem or leaf, so that protection is gained by 

 their close resemblance to the flower-buds of the stin^incr- 

 nettle, which forms the food-plant of their caterpillars? 



Of course the butterfly is not aware of the advantage which 

 follows from such a proceeding ; intelligence has no part in the 

 process. The entire operation depends upon certain inherent 

 anatomical and physiological arrangements : — on the structure 

 of the ovary and oviducts, on the simultaneous ripening of a 

 certain number of eggs, and on certain very complex reflex 

 mechanisms which compel the butterfly to lay its eggs on 

 certain parts of certain plants. Schneider is certainly right 

 when he maintains that this mechanism is released by a sen- 

 sation, arising from the perception (whetlier by sight or smell, 

 or both together) of the particular plant or part of the plant 

 upon which the eggs are to be laid ^ At any rate, we cannot, 

 in such cases, appeal to the effects of constant use and the 

 transmission of acquired characters, as an explanation ; and 

 the origin of the impulse can only be understood as a result of 

 the process of natural selection. 



The protective cocoons by which the pupae of many insects 

 are surrounded also belong to the same category, and improve- 

 ment by practice is entirely out of the question, for they are 

 only constructed once in the course of a life-time. And yet 

 these cocoons are often remarkably complex : think, for in- 

 stance, of the cocoon spun by the caterpillar of the emperor 

 moth {Safurnia carpini), which is so tough that it can hardly 

 be torn, and which the moth would be unable to leave, if an 

 opening were not provided for the purpose ; while, on the 

 other hand, the pupa would not be defended against enemies 

 if the opening were not furnished with a circle of pointed 

 bristles, converging outwards, on the principle of the lobster 

 pot, so that the moth can easily emerge, although no enemy 

 can enter. The impulse which leads to the production of such 

 a structure can only have arisen by the operation of natural 

 selection— not, of course, during the history of a single species, 



^ Compare Schneider, ' Der thierische Wille.' 



