24 



succession of broods, of eight larvae, like itself, without any 

 connection with the male. In fact no winged males at this 

 season have appeared. If the virgin progeny be also kept 

 from any access to the male, each will again produce a 

 brood of the same number of aphides : and carefully pro- 

 secuted experiments have shown that this procreation from 

 a virgin mother will continue to the seventh, the ninth, or 

 the eleventh generation before the spermatic virtue of the 

 ancestral coitus has been exhausted. 



When it is so exhausted, a greater proportion of the 

 nuclear germ-masses retained by the last procreant larvae 

 is used up : individual growth and development proceed 

 further than in the parent : some members of the last larval 

 brood are metamorphosed into winged males, others into 

 oviparous females. By these the ova are developed, impreg- 

 nated and oviposited, and thus provision is made for dis- 

 seminating the individuals and for continuing the existence 

 of the species over the severe famine-months of winter. 



The metamorphosis in all insects is attended with the 

 casting off or creeping out of a certain proportion of the 

 precedent individual, which process is called the 'ecdysis* 

 or moult. With regard to the so-called metamorphoses 

 which issue in the succession of a fixed, blind, sessile 

 multivalve barnacle to a free-swimming crustacean w T ith 

 pedunculated eyes*, or in the succession of a rooted 

 vermiform parasite to a natatory animal with articulated 

 setigerous limbs t when these phaenomena are closely 

 traced, they are seen to depend in a greater degree 

 upon the action and coalescence of retained cells, than 

 upon a change of pre-existing tissues. If the development 

 of the ovum in the pedunculate ovarian sac of the low 

 organized crustaceous epizoon of a fish be closely traced, 

 the peripheral cells of the germ-mass are seen to combine 

 and coalesce to form the smooth transparent skin of the 



* J. V. Thompson, Zoological Researches, 8vo, p. 69. 

 f Norclmann, Mikrographische Beitrage, Heft ii. 1832. 



