324 THE STUDY OF ANIMAL LIFE CHAP. 



often differ from one another as different forms of a 

 melody do when the " time ' of the various parts is 

 altered, and this change of rate is often finely adapted to 

 particular conditions, that is to say, it furnishes a solution 

 of special problems of life. The morphologists are 

 beginning to discern that one type of skull, or one shape 

 of fish, or one contour of leaf, may be derived from another 

 by supposing a slight deformation a tilting of axes, or 

 an alteration of the angles at which the dominant lines 

 meet and an idea to be kept in mind in studying life- 

 histories is that one creature's often differs from another's 

 in a change of rate or rhythm, in an elongation of one part 

 of the life-curve and a compression of another part. 



2. Larval Periods. The developing creature is called 

 an embryo so long as it is within the egg-shell or egg- 

 envelope. If what emerges from the egg is in the main 

 a small edition of the parent, as in the case of a chick, we 

 call it a young or juvenile creature ; but if what emerges 

 is built on different lines from the parent, we call it a 

 larva, as in the case of a tadpole. The larva requires 

 to undergo some degree of metamorphosis before it puts 

 on the form of the adult ; thus the life-historv shows some 



* v 



circuitousness. This is very striking in cases where 

 one larval form succeeds another, as in the Crustacean 

 Penceus discussed in the last chapter. 



A familiar kind of life-history is that into which a 

 prolonged larval period has been interpolated. Out of 

 the egg-shell of a cockroach or an earwig there comes a 

 tiny creature which is in most respects a miniature of the 

 adult, but out of a butterfly's egg there emerges a minute 

 caterpillar which docs not give much hint of its parentage. 

 It feeds and grows and moults its cuticle, and this logical 

 sequence is repeated over and over again. The cater- 

 pillar gains strength and stores up nutritive reserves 

 the so-called fatty body ; it undergoes a remarkable 

 metamorphosis, most of the larval body breaking down 

 and a fresh start in development being made on a new 

 architectural plan. After a period of quiescence (the 

 pupa, or nymph, or chrysalis), which is often prolonged, 

 the winged butterfly (the imago) emerges, as if by a 



