The Zoologist— August, 1807. 847 



There are two Vanessa; on the Continent totally different in general 

 colouring, superficial appearance and season of flight : they are de- 

 scribed by Linneus, Fabricius and other authors of unquestioned 

 authority, under the name of Levana and Prorsa ; they are figured in 

 Schaffer's ' Jcones lnsectorum circa Ratisbouara indigenorum ;' Levana 

 in pi. 217, figs. 3 and 4, Prorsa in pi. 132, figs. 1 and 2; and again in 

 Hubner's ' Samlung Europaischer Schmetterlinge,' Levana, figs. 97 

 and 98 ; Prorsa, figs. 94, 95 and 96. These four great entomologists 

 give the two insects as perfectly distinct, nor do they allude to any 

 similarity between them. Levana appears on the wing in April, 

 Prorsa in June and July. In 1843 Herrich-Schaffer's magnificent 

 'Schmetterlinge von Europa' appeared, and in this we find the 

 intimation that Levana and Prorsa are the two moieties of one species ; 

 but notwithstanding this knowledge he gives both species, evidently 

 considering the discovery too startling to be allowed to pass un- 

 challenged. " That Prorsa and Levana really constitute but a single 

 species," says Herrich-Schaffer, "is now perfectly established, but 

 nevertheless I consider it best to allow both names to remain/' as 

 though he could not allow this astounding fact to appear all at once 

 in its naked truth before the entomological world. It is now universally 

 received by continental authors, and the only point proposed to be 

 examined is whether the pupae of Levana could by subjection to a low 

 temperature be induced to remain in that state during the autumn and 

 winter, and whether they would in the following April produce Levana 

 or Prorsa. Again, whether all the alternating species complete two 

 cycles or two generations in the course of every year, and whether all 

 double-brooded insects are alternating, are questions which require 

 the most careful investigation, but questions on which I will not 

 presume to enter. 



Another law, that of agamic reproduction, or agamogenesis, has 

 been unfortunately so mixed up with this of atavism in the case of the 

 Aphides or plant-lice, that it has become almost impossible to separate 

 the two, and give to each its due importance ; and here it may be ob- 

 served that alternate generations are not unfrequenlly found to be 

 agamic ; but this would lead us into a field of inquiry too extensive 

 for this essay. The winged Aphis, at the end of summer, leaves for 

 ever the plant on which it has been nourished ; it is borne on the wind 

 without exhibiting any symptoms of volition; countless myriads perish, 

 but some, a very small proportion, are driven against trees or shrubs, 

 the sap of which will suit their descendants. The female when thus 



