M. Leuckart on the Reproduction of Bark-lice. 421 



A second circumstance, of great importance in the inquiry 

 into the nature of the reproduction of the Aphides, is that the 

 gvnn-gramdes of these animals are only developed in certain 

 individuals, whilst other individuals, making their appearance 

 under particular circumstances, lay unmistakeable eggs, and 

 fertilize these in the usual manner. 



From the point of view of the alternation of generations, such 

 a regular alternation of fertilized and unfecundated individuals 

 appears quite natural, and even necessary ; but in the domains of 

 Parthenogenesis, as far as our present knowledge goes, we seek in 

 vain for an analogous case. Although in the Bark-lice, mLecanium 

 Hesperidum, Solcnobia Hchenella, and other animals which usually 

 propagate exclusivelv by Parthenogenesis, a fecundation (by the 

 males, which are still entirely unknown to us) may perhaps take 

 place from time to time, there is still not the least ground for 

 assuming a regular, and, under certain circumstances, necessary 

 repetition of this process. In all these cases we must take the 

 possibility of a fecundation into consideration for every distinct 

 individual; a fecundation which takes place only occasionally, 

 and then necessarily, in particular individuals, but is and must 

 be just as regularly dispensed with in the intervals, has been 

 recognized at present only amongst animals which are developed 

 in accordance with the alternation of generations. 



If, in spite of all these considerations, the reproduction of the 

 Aphides is to be ranged under Parthenogenesis*, we are com- 

 pelled, as, indeed, Claus very justly felt, to establish a peculiar 

 form for it. This would then stand in precisely the same rela- 

 tion to the ordinary Parthenogenesis, in which each individual 

 produces eggs capable of spontaneous development, as the alter- 

 nation of generations to ordinary asexual reproduction, which, 

 as is well known, in the animals with an alternation of genera- 

 tions, is likewise transferred only to particular individuals, pecu- 

 liarly organized for the purpose. 



The Parthenogenesis of the Aphides would thus still remain 

 to a certain extent allied to the alternation of generations, even 

 if it did not positively coincide therewith. 



We must leave it to future observers to decide positively as to 

 the justice of one or other of these views. Our observations upon 

 Parthenogenesis are still so recent, and the possibilities of the 

 differences and combinations occurring here so little known and 

 weighed, that it appears scarcely possible at present to formulize 



* It may be mentioned here, in passing, that, from analogy with the 

 Aphides, the propagation of Gyrodactylus elegans must also be regarded 

 as Parthenogenesis. Here also there is the same analogy between the 

 nurses and sexual animals, the brood-stock and germ-stock, the germ-cell 

 and the egg (germinal vesicle). 



