INTRODUCTION 9 



As soon as the cremaster is entangled in the web the cast 

 skin usually falls off and for a very short period the creat- 

 ure hanging seems to be neither caterpillar nor chrysalis. 

 It is in fact in a transition stage between the two, and it 

 very soon shortens up and takes on the definite form of the 

 chrysalis, the outer tissues hardening into the characteris- 

 tic chrysalis skin. 



From the fact that this chrysalis skin shows man}'^ of the 

 cliaracteristic featiu'es of the future butterfly it is evident 

 that the change from the caterpillar to the butterfly really 

 began during the life of the larva. The nature of the 

 process by which this change takes place has long been a 

 puzzle to scientists. For the making of a butterfly is one 

 of the most wonderful phenomena in the outer world, and 

 it has challenged the attention of many acute observers. 

 Some two centuries ago the great Dutch naturalist, Swam- 

 merdam, studied very carefully the development of many 

 insects, especially the butterfly. He found that if he 

 placed in boiling water a caterpillar that was ready to 

 pupate or become a chrysalis, the outer skin could easily be 

 removed, revealing beneath the immature butterfly with 

 well-developed legs and antennae. From these observa- 

 tions he was led to believe that the process of growth was 

 simply a process of unfolding; that is, as Professor Packard 

 has expressed it, "That the form of the larva, pupa, and 

 imago preexisted in the egg and even in the ovary; and that 

 the insects in these stages were distinct animals, contained 

 one inside the other, like a nest of boxes or a series of en- 

 velopes one within the other." This was called the in- 

 casement theory and it was held to be correct by naturalists 

 for nearly a century. It was discredited, however, 

 about a hundred years ago, but not until another fifty 



