310 PHYSIOLOGY. 



duce only such organisms that are adapted to the organ upon which 

 the germ has formed itself. For the skin these are parasitic insects, and 

 consequently only such, viz. lice, can be produced ; beneath the skin, 

 however, the parasitic arachnids; (Acarinee) originate precisely in the 

 same manner. In the pustules of the itch they are developed only so lonj; 

 as they themselves are forming, and therefore containing lymph. We 

 may therefore consider that it is from this lymph that their germs are 

 developed; subsequently, however, when the material producing the germ 

 is exhausted, the lymph itself corrupts, and becomes pus. Precisely 

 the same takes place in the Endozoa. Von Bar has observed this deve- 

 lopment in the remarkable Bucephalus, and it is as good as proved in 

 many others ; why should not therefore the skin, which has precisely the 

 same structure as the mucous membrane of the intestinal canal, give 

 rise also to parasites peculiar to it? I know nothing that satisfactorily 

 opposes the adoption of it. Equivocal generation consequently takes place 

 in the lowest insects : they can originate from it, and do so frequently. 



204. 



The second kind of propagation, that by shoots, has not yet been 

 observed in insects ; it is also perfectly contradictory to the idea of 

 creatures so highly organised they are. Some observations, however, 

 seem to confirm the possible development of insects from germens or 

 eggs laid by an unimpregnated female. We will here communicate 

 these instances. 



All observations hitherto made upon this subject may be divided 

 into two groups, the one of which seems to prove that this mode of 

 propagation constantly and regularly takes place in certain genera, and 

 the other that it occurs but occasionally, and as exceptions. As a 

 regular mode of propagation, it is ascribed to the Aphides, or plant lice. 

 These produce throughout the whole summer living female young ones, 

 which again, without any preceding impregnation, according to the obser- 

 vations of De Geer and Bonnet, also produce living female young ones. 

 This spontaneous development is repeated to the tenth generation, and 

 indeed still further, if, as Kyber has proved by experiment, the plant 

 lice with the plants they inhabit be removed into heated rooms to pass 

 the winter. Treated thus, Kyber observed a colony of Aphis Dianl/ti 

 continue to propagate for four years without the single impregnation of 

 a femak 1 by a male, but they continued to produce young ones which 



