378 MENTAL EVOLUTION IN ANLMALS. 



afterwards disengages and casts off the skin which had served 

 it for a hidder.* I am tempted to give one other analogous 

 case, that of the caterpillar of a Butterfly {ThcMa), which 

 feeds within tlie pomegranate, but when full fed gnaws its 

 way out (thus making the exit of the butterfly possible before 

 its wings are fully expanded), and then attaches with silk 

 threads the point to the branch of the tree, that it may not 

 fall before the metamorphosis is complete. Hence, as in so 

 many other cases, the larva works on this occasion for the 

 safety of the pupa and of the mature insect. Our astonish- 

 ment at this manoeuvre is lessened in a very slight degree 

 when we hear that several caterpillars attach more or less 

 perfectly with silken threads leaves to the stems for their 

 own safety ; and that another caterpillar, before changing 

 into a pupa, bends the edges of a leaf together, coats one 

 surface with a silk web, and attaches this web to the foot- 

 stalk and branch of the tree; the leaf afterwards becomes 

 brittle and separates, leaving the silken cocoon attached to 

 the footstalk and branch ; in this case the process differs but 

 little from the ordinary formation of a cocoon and its attach- 

 ment to any object. f 



A really far greater difficulty is offered by those cases in 

 which the instincts of a species differ greatly from those of 

 its related forms. This is the case with the above mentioned 

 Thekla of the pomegranate; and no doubt many instances 

 could be collected. But we should never forget what a small 

 proportion the living must bear to the extinct amongst 

 insects, the several orders of which have so long existed on 

 this earth. Moreover, just in the same way as with corporeal 

 structures, I have been surprised how often when I thought 

 I had got a case of a perfectly isolated instinct, I found on 

 further enquiry at least some traces of a graduated series. 



I have not rarely felt that small and trifling instincts 

 were a greater difficulty on our theory than those which 

 have so justly excited the wonder of mankind ; for an 

 instinct, if really of no considerable importance in the 

 struggle for life, could not be modified or formed through 

 natural selection. Perhaps as striking an instance as can be 

 given is that of the w^orker of the Hive-bee arranged in files 

 and ventilating, by a peculiar movement of their wings, the 



* Kirbj and Spence, JEntomology, voL iii, pp. 208-11. 

 t J. O. Westwoocl in Trans. Entomol. Soc, vol. ii, p. 1. 



