352 GENERAL PRINCIPLES 



must have been a process of absorption at work as development 

 proceeded. 



738. Progressive and Regressive Development. — Wherever 

 development is direct the organization of the body is a con- 

 tinuous progressive process toward the final perfect adult. But 

 when there is a change in the course of development, as in all 



Fig. 219. — Sexual dimorphism in a beetle, Cladognathus. The difference 

 appears both in size and in the peculiar development of the mandibles of the 

 male. Male on the left. In many beetles the male is larger than the female. 

 X5/6. 



cases of metamorphosis, or where there is a radical change in 

 the life habits of the animal, there is also a break in the con- 

 tinuity of development, and to a greater or lesser degree a re- 

 versal of development. This is the case, for example, when a 

 free-swimming larva becomes fixed in the adult, or when a holo- 



