156 THE EVOLUTION THEORY 



To this class I kK )n^- .ill the diverse instincts by which insects protect 

 themselves against attack during the pupa stage. Even the way in 

 which the caterpillars of many diurnal butterflies hang themselves up 

 in pupation is not by any means a very simple instinctive action. 

 The caterpillar first spins, in a suitable place, a small round disk 

 of silk threads, to which it then attaches the posterior end of its 

 body, so securely that it cannot be easily torn away. More compli- 

 cated still is the securing of the pupa when it does not hang freely, 

 " but is to remain pressed against a wall or a tree, as is the case in the 

 Papilionida? and the Pieridie. In this case the caterpillar must, in 

 addition to the usual cradle, spin a thread of silk, in an ingenious 

 wa}^ diagonally across the thorax, so that it may cross about the 

 middle of the wing rudiments, and not be too loose, lest the pupa fall 

 out, yet not too tight, lest the thread cut too deeply into the wing 

 rudiments and hinder their development. When one remembers that 

 it is the caterpillar that does all this, before it has taken the form 

 of the pupa, and that it must all be adapted to the pupa's form, we 

 are amazed at the extraordinary exactness with which instinct 

 prescribes all the individual movements which make the whole of the 

 complex performance effective. And yet, as each caterpillar only 

 accomplishes this performance once in its life, it could at no time 

 in the development of the species have become a habit in the case of 

 any individual caterpillar, and it cannot therefore be an ' inherited 

 habit.' 



But however diverse are the methods of securing the safety of 

 the pupje in the different families of butterflies, they must all be 

 referred back to a single root, if the butterfly pedigree can be traced 

 back to a single ancestral group. The caterpillar of the Sphingid^e 

 does not creep uj) walls and trees when it is ready to enter on the 

 pupa stage, as so many of the caterpillars of the diurnal butterflies do, 

 but instead its instinct compels it to run about on the ground until it 

 has found a spot which seems to it suited for boring into the earth, 

 or, to speak less metaphorically, until it comes to a place which, from 

 its nature, acts as a liberatinp; stimulus to the instinct to burrow. 

 Then it penetrates more or less deeply, according to the species, and 

 makes a small chamber, which it lines with silken threads to prevent 

 it collapsing ; this done, it moults, and enters on the pupa stage. 

 The exactness with which the individual movements are prescribed 

 by instinct is seen in the wp-y in which the size of the chamber is 

 regulated so as to be exactly as large as is necessary to give the pupa 

 room enough without leaving any superfluous free space. This is not 

 so simple as it seems, and is not directly conditioned by the size of the 



