CAUSES OF METAMORPHISM 705 



SUMMARY OF THE FACTS AND SUGGESTIONS AS TO 

 THE CAUSES OF METAMORPHISM 



An explanation of the causes of metamorphosis is one of the 

 most difficult undertakings in biology, and the phenomenon has 

 been considered as one of the chief difficulties in the way of the 

 acceptance of the theory of descent. 



A review, however, of the facts of hypermetamorphism, particu 

 larly the life-history of Mantispa, throws much light on the subject, 

 since it is very probable that the supernumerary stages and marked 

 changes of form characterizing them are due to changes of environ- 

 ment, of habits, and of food, causes which have exerted such a pro- 

 found influence on organic beings throughout all time. Besides 

 these, as the result of changes in the environment and nature of the 

 food, we have the results brought about by the use or disuse of 

 structures brought into existence by the action of stimuli from with- 

 out, the class of insects abounding in examples of temporary struct- 

 ures which perform a certain function, and then disappear. 



Again, if the origin of a hypermetamorphosis can thus be ex- 

 plained, it follows that normal metamorphosis is most probably due 

 to changes of habitat, of seasons, of food, and to accelerated growth 

 resulting from the approach of sexual maturity. 



The following facts and conclusions appear to be well estab- 

 lished : 



1. The apterous insects (Synaptera) are ametabolous, only the 

 winged insects undergoing a metamorphosis. 



2. The complete metamorphosis was not inherited from the prim- 

 itive ancestor of all insects, but acquired at a later period (F. Muller). 

 The cruciform type is a secondary, adaptive form, derived from the 

 earlier, campodeoid type of larva. 



3. The earliest, most primitive pterygote insects passed through 

 only a slight metamorphosis. In other words, as soon as the wings 

 were evolved and insects became adapted to live or take refuge in a 

 new medium, the air, at the approach of the period of adult life, 

 with the ripening or perfection of the reproductive organs, a meta- 

 morphosis began to take place, and the number of species greatly 

 multiplied. On the other hand, the Arachnida and Myriopoda, in 

 which as a rule there is no metamorphosis, being confined to a creep- 

 ing life, with no change of medium, remained poor in number of 

 species. 



4. At first the nymphs mainly differed from the adults in lacking 



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