i] INTRODUCTION 7 



segmented worm and an oyster, unlike enough when 

 adult, develop from closely similar larval forms. If we 

 take a class of animals, the Crustacea, nearly allied to 

 insects, we find that its more lowly members, such as 

 'water-fleas' and barnacles, pass through far more 

 striking changes than its higher groups, such as 

 lobsters and woodlice. But among the Insects, a 

 class of predominantly terrestrial and aerial creatures 

 producing large eggs, the highest groups undergo, as 

 we shall see, the most profound changes. The life- 

 story of the butterfly, then, well-known as it may be, 

 furnishes a puzzling exception to some wide-reach- 

 ing generalisations concerning animal development. 

 And the student of science often finds that an 

 exception to some rule is the key to a problem of 

 the highest interest. 



During many centuries naturalists have bent their 

 energies to explain the difficulties presented by insect 

 transformations, Aristotle, the first serious student 

 of organised beings whose writings have been pre- 

 served for us, and William Harvey, the famous 

 demonstrator of the mammalian blood circulation 

 two thousand years later, agreed in regarding the 

 pupa as a second egg. The egg laid by a butterfly 

 had not, according to Harvey, enough store of food 

 to provide for the building-up of a complex organ- 

 ism like the parent ; only the imperfect larva could 

 be produced from it. The larva was regarded as 



