ASPECTS OF KINETIC EVOLUTION 24I 



Metamorphic differences are largely adaptive, but it is none 

 the less probable that the alternation of bodily forms and the 

 change of food and environment may contribute something to 

 the same physiological results as diversity of descent. In the 

 more specialized insects metamorphosis is accompanied by a 

 complete disorganization of the larval tissues, the pupae repre- 

 senting, as it were, a return to the egg stage, the change of ex- 

 ternal form affording an opportunity for a complete rebuilding 

 of the cellular structure of the body. It may be that this fact, 

 viewed in connection with the extremely complex nuclear organs 

 of the cells of insects, will assist in explaining the unique effi- 

 ciency of the insect organism. 



Metamorphosis is not restricted, however, to animals. In 

 plants like Eucalyptus and Junificrus there are sudden changes 

 of form and structure from the juvenile to the adult phase of the 

 species. 



HETERCECISM. 



Many plant and animal parasites infest two or more hosts in 

 different stages of their life-history. Changes of hosts are then 

 usually coincident with metamorphoses, or with change of gen- 

 eration or of structural phases. It has been inferred by some 

 that the abrupt change in the organism is due to the change of 

 food and other conditions of existence, but this does not find 

 confirmation in the studies of the life-histories of the parasites. 

 The indications are more favorable to the opposite suggestion 

 that the great diversity of conditions has enabled the parasites 

 to proceed on two or more independent courses of evolution. 



The parasites have developed the power of living in two or 

 three distinct environments at different periods of their life- 

 history, and the characters which adapt them to this variety of 

 conditions have been attained, apparently, in quite the same 

 manner as the characters of other less specialized plants and 

 animals. 



The more primitive simple-celled stage, or haplogamic 

 phase, of many species of rust-fungi is confined to pines or to 

 others of the more primitive families of plants, while the more 

 advanced and efficient double-celled phase of the parasite has 

 been able to attack plants of more highly developed families, 



