23 



second young annelide is formed: sometimes a third, a 

 fourth, a fifth, and even a sixth may be generated, and thus 

 seven individuals may be seen organically connected in a 

 linear series*. They are ultimately separated by sponta- 

 neous fission. The gemmiparous parent has no generative 

 organs: these are developed in its offspring, which pro- 

 pagate by impregnated ova, producing in the Nereids a 

 ciliated monadiform embryo, metamorphosed into the seti- 

 gerous worm, which produces by gemmation and sponta- 

 neous fission the true males and females of its species f. 



Closely analogous to this alternating mode of reproduc- 

 tion, and to that cyclical succession of differently formed 

 individuals which has been adduced from the Medusa and 

 the Distoma, are the generative phenomena presented by 

 certain members of the class of Insects, in which many 

 individuals are the product of a single impregnated ovum, 

 and are produced in successive generations from the indi- 

 vidual proceeding from that ovum. 



It is now more than a century since Bonnet} first at- 

 tracted the attention of physiologists and naturalists to 

 this mode of generation in the Aphides or Plant-lice. And 

 because it was the first of a large class of phenomena, till 

 then utterly unknown and unsuspected, it was received 

 with considerable doubt, or met by total incredulity. 



The facts are briefly these. 



The impregnated ova of the Aphis are deposited, at the 

 close of summer, in the axils of the leaves either of the 

 plant infested by the species or of some neighbouring 

 plant, and the ova retaining their latent life through the 

 winter, are hatched by the returning warmth of spring: 

 a wingless hexapod larva is the result of the develop- 

 ment. This larva, if circumstances, such as warmth and 

 food, be favourable, will produce a brood, and indeed a 



* Milne-Edwards, 'Annales des Sciences/ 1845, pi. 11. fig. 65. 



f Ibid. p. 170. 



% ' Traitc d'Insectologie, ou Observations sur les Pucerons/ 8vo, 1745. 



