708 TEXT-BOOK OF ENTOMOLOGY 



paralleled by those of the Amphibia. The case of paedogenesis of 

 Chironomus affords a parallel with that of the Siredon, or larva of 

 Amblystoma. Also the organs and appendages of the insects, such 

 as caterpillars, are present, just as the skeleton and other organs of 

 the tadpole are the homologues of those of the adult, although these 

 parts undergo a profound modification, and new structures are added. 

 (See the discussion of this point by Miall, and by Hyatt and Arms.) 



Theoretical conclusions ; Causes of metamorphosis. It results from 

 a review of the known facts, together with reasonable inductions 

 from such facts, that so far from opposing the theory of descent, the 

 facts of metamorphosis, and particularly of hypermetamorphosis, 

 seem to afford solid foundation for the theory. While natural selec- 

 tion was not the initiative cause, it plays a part as one of several 

 factors ; but the fundamental causes are the same as those which 

 have controlled the origin of species and of the larger groups of 

 animals in general. Owing to the struggle for existence, due to 

 overcrowding, the early insects were forced to take to the air, acquir- 

 ing wings to enable them to avoid the attacks of creeping and run- 

 ning insects. In the end the insects became, owing to this acquisition 

 of wings, and afterwards to the establishment of a complicated 

 metamorphosis, numerically the most successful type of life in exist- 

 ence, the number of species, extinct and living, mounting into the 

 millions. 



All aquatic insects are evidently the descendants of terrestrial 

 forms, and the numberless contrivances and temporary larval organs, 

 particularly of dipterous larvae, are evidently adaptations to the 

 needs of the insect during its aquatic life, and which are cast aside 

 when the creature passes to a different medium. The sudden or 

 tachygenic appearance of temporary structures, such as hatching 

 spines, various setae, spines, respiratory organs, so characteristic of 

 dipterous larvae, and of the protective colors and markings of cater- 

 pillars, and which are discarded at pupation, or imagination, are 

 evidently due to the action of stimuli from without, to the primary 

 neolamarckian factors, the characters proper to each larval stadium, 

 and to the pupal and imaginal stadia, characters probably acquired 

 during the lifetime of the individual, becoming finally fixed by 

 homochronous heredity. 



