322 ON THE- PHENOMENA OF GEMMATION 



for three years in uninterrupted succession ; and the males and true 

 oviparous females of the A. diantJii have never yet been met with. 

 The current notion that there is a fixed number of broods, " nine or 

 eleven," is based on a mistake. 



As, under moderately favourable conditions, an Aphis comes to 

 maturity in about a fortnight ; and as each Aphis is known to be 

 capable of producing a hundred young, the number of the progeny 

 which may eventually result even from a single Aphis during the six 

 or seven warm months of the year is easily calculated. M. Tougard's 

 estimate adopted (and acknowledged) by Morren, and copied from 

 him by others, gives the number of the tenth brood as one quintillion. 

 Supposing the weight of each Aphis to be no more than y^jVcrth of a 

 grain, the mass of living matter in this brood would exceed that in 

 the most thickly populated countries in the world. 



The agamogenetic broods are either winged or wingless. The 

 winged forms at times rise into the air, and are carried away by the 

 wind in clouds ; and these migrating hordes have been supposed to 

 be males and females, swarming like the ants and bees ! During the 

 summer months it is unusual to meet other than viviparous Aphides, 

 whether winged or wingless ; but ordinarily, on the approach of cold 

 weather, or even during warm weather, if the supplies of food fall 

 short, the viviparous Aphides produce forms which are no longer 

 viviparous, but are males and oviparous females. The former are 

 sometimes winged, sometimes wingless. The latter, with a single 

 doubtful exception, are always wingless. 



The oviparous females lay their eggs, and then, like the males, die. 

 It commonly happens also that the viviparous ApJiides die, and then 

 the eggs are left as the sole representatives of the species ; but in 

 mild winters many of the viviparous Aphides merely fall into a state 

 of stupor and hybernate, to re-awake with the returning warmth of 

 spring. At the same time the eggs are hatched and give rise to 

 viviparous Aphides, which run through the same course as before. 

 The species Aphis, therefore, is fully manifested not in any one being or 

 animated form, but by a cycle of such, consisting of, — ist, the egg ; 2nd, 

 An indefinite succession of viviparous Aphides ; 3rd, Males and females 

 eventually produced by these, and giving rise to the egg again. 



If, armed with the microscope and scalpel, we examine into the 

 minute nature of these processes (without which inquiry all specula- 

 tion upon their nature is vain), we find that the viviparous Apliis 

 contains an organ similar to the ovarium of the oviparous female, in 

 some respects, but differing from it, as Von Siebold was the first to 

 show, in the absence of what are termed the colleterial glands and the 

 spermatheca — organs of essential importance to the oviparous form. 



