REGRESSIVE DEVELOPMENT 353 



zoic larva becomes paraistic in the adult. Such changes in- 

 volve a loss of function, or at least a change of function of some 

 organs, and hence a change in the organs themselves. This is 

 called regressive development. When the tadpole develops 

 legs and lungs and leaves the water, some of its organs have 

 become useless. We need mention only the gills and the broad 

 fish-like tail. These organs, being now no longer needed, un- 

 dergo regressive changes, they are gradually resorbed, dwindle 



FIG. 220. Male (left) and female (right) of a fire-fly, Lampyris. The male has 

 well-developed wings but the female is wingless. X2. 



and completely disappear. It must not be inferred, however, 

 that if any tadpole were kept in the water that these changes 

 would not occur. Indeed, these organs have become useless 

 before the frog leaves the water. The position might be assumed 

 that the change of habit occurs because of the change in 

 organization. (See p. 339.) 



739. Sexual Dimorphism. In some species the adult in- 

 dividuals all strictly conform to one type. This is exceptional, 

 however, and applies only to hermaphrodyte forms like the 

 earthworms and some snails. The vastly more common con- 

 dition is a sexual dimorphism; that is, two types of individuals 

 23 



