ORIGIN OF A METAMORPHOSIS. 181 



so-called pupae, or half winged individuals known not to be 

 adult, in some cases feel the sexual impulse, while a number of 

 species in each of the families represented by these two insects 

 never acquire wings. 



Still how did the perfect metamorphosis arise? We can only 

 answer this indirectly by pointing to the Panorpa and Caddis 

 flies, with their nearly perfect metamorphosis, though more 

 nearly allied otherwise to those Neuroptera with an incomplete 

 metamorphosis, as the lace-winged fly, than the insects of any 

 other suborder. If, among a group of insects such as the Neu- 

 roptera, we find different families with all grades of perfection 

 in metamorphosis, it is possible that larger and higher groups 

 may exist in which these modes of metamorphosis may be fixed 

 and characteristic of each. Had we more space for the exposi- 

 tion of many known facts, the sceptic might perceive that by 

 observing how arbitrary and dependent on the habits of the 

 insects are the metamorphoses of some groups, the fixed modes 

 of other and more general groups may be seen to be probably 

 due to biological causes, or in other words have been acquired 

 through changes of habits or of the temperature of the seasons 

 and of climates. Many facts crowd upon us, which might serve 

 as illustrations and proofs of the position we have taken. For 

 instance, though we have in tropics rainy and dry seasons when, 

 in the latter, insects remain quiescent in the chrysalis state as in 

 the temperate and frigid zones, yet did not the change from the 

 earlier ages of the globe, when the temperature of the earth 

 was nearly the same the world over, to the times of the present 

 distribution of heat and cold in zones, possibly have its influence 

 on the metamorphoses of insects and other animals? It is a 

 fact that the remains of those insects with a complete meta- 

 morphosis (the bees, butterflies and moths, flies and beetles) 

 abound most in the later deposits, while those with an incom- 

 plete metamorphosis are fewer in number and the earliest to 

 appear. Again, certain groups of insects are not found in the 

 polar regions. Their absence is evidently due to the adverse 

 climatic conditions of those regions. The development of the 

 same groups is striking in the tropics, where the sum of envi- 

 roning conditions all tend to favor the multiplication of insect 

 forms. 



It should be observed that some insects, as the grasshopper, 

 for example, as Miiller says, "quit the egg in a form which is dis- 

 16 



