10S BRITISH APHIDES. 



the Protozoa multiplication takes place by fission or 

 separation into two or more individuals ; or by budding 

 or gemmation, where a separation of the smaller indi- 

 vidual from its parent is preceded by a growth in 

 the lesser form. According to Prof. Balfour, no 

 essential portion of the parent is removed during 

 this process. A portion of a nucleus may separate 

 either externally or internally from the protoplasm. 

 The spores of animals may break up into very 

 numerous parts, all of which may materially differ 

 from the form of their parents. In the Infusoria such 

 spores are often furnished with a vibratile lash or 

 flagellum, by which they progress through the water 

 or other fluids in which they live. 



Even in the lowest vegetable forms a conjugation 

 of cells seems to be the condition of a renewed vitality; 

 though the fact is remarkable, that the conjugation of 

 two or more individuals does not invariably lead to 

 immediate reproduction. Such conjugation, however, 

 probably conduces to a fresh vigour in the animal, and 

 conduces to a differentiation which checks the evils of 

 self -fertilisation. 



In the Metazoa both sexual and non-sexual pheno- 

 mena obtain. The first consists of the fusion of the 

 ovum and the spermatozoon, attended by a subsequent 

 division of the compound cell. Development hero 

 mostly results in the production of forms similar to 

 the parents. 



La Hire, perhaps, was the first to observe the process 

 of egg-layingin Aphis ; and al fclie same time he demon- 

 strated that both viviparism and oviparism might go 

 on side by side in the same family of insects. 



Prof. Owen, in more recent times, revived the theory 

 originally started, it is believed, by Dutrochct, in a 

 lecture delivered to the members of the Royal Institu- 

 tion of London. After giving an interesting definition 

 of the collective individual, Prof. Owen stated thai there 

 was mi scientific diiHculty in conceiving the efficacy 

 of the male influence as shown by Aphis, notwith- 



