90 GENERAL EVOLUTIOK 



completely developed." ''I think," says Spinola, "that the pres- 

 ence of wings and their development depends on the climate" ; and, 

 in speaking of Oncocephalus griseus, he says, " The influence of 

 the northern climate appears to have arrested the development of 

 the organs of flight. It will be seen that I have referred else- 

 where that I have noticed that it is especially in hot seasons that 

 certain species acquire wings, while the circumstance noticed re- 

 specting the ordinary occurrence of winged specimens of Micro- 

 coelia in the West Indies is confirmatory of the same opinion." 



4. It is now known that certain Orthoptera do not get through 

 their metamorphosis in time for the period of reproduction, and 

 hence never, or in rare instances only, develop more than a short 

 distance beyond the pupa state. 



5. My friend P. R. Uhler tells me of an example among 

 Hemiptera of the genus Velia. The species V. rivulorum, Fab., 

 and V. curreiis,^ of Europe, are only distinguished by the devel- 

 opmental feature of the presence of wings in one, and their ab- 

 sence in the other. Another species of the tropical region of the 

 West Indies, Halobates americanus, Uhler, is furnished with 

 wings, while its individuals, which occur abundantly in North 

 America, have been generally supposed to lack them. Individuals, 

 however, no doubt occur whose development is so far accelerated 

 as to permit them to acquire wings before the period of rej^roduc- 

 tion, since one such has been found by Uhler. 



These wing characters are in many cases generic, it aj^pears to 

 the writer ; and the fact that they differ, without corresponding 

 specific differences, is important evidence as to the origin of the 

 genera. 



6. The females of the Lepidopterous genus Thyridopteryx 

 never develop beyond the pupa state, according to the same au- 

 thority, before reproduction ; they are reproducing pupae, so far 

 as the external characters concerned in metamorphosis go. In 

 other words, the latter have been retarded, while the reproductive 

 system and others have progressed. Now, generic characters are 

 seen in the first, not in the last. The influence of the males is 

 sufficient to prevent more than a part of the offspring from being 

 retarded in the same manner. 



I have selected a few of this class of facts which have come 

 before my mind during the present writing, as drawn mainly from 



* On " Insects," ii, p. 493, 



