Royal Institution. 61 



original condition, as far as history gives us any knowledge of it. 

 With each successive generation the series of changes recommences 

 from the old point, and ends in a condition of the animal adapted to 

 set the same series again on foot. 



Having traced the principal stages in the metamorphosis of an 

 animal from a swimmer to a leaper, the Lecturer next took an instance 

 where one begins life as a burrower or a crawler, and is converted into 

 an animal of rapid and powerful flight. 



Most insects quit the egg in the form of a worm, which masking, 

 as it were, a different and higher form, is called the * larva ' ; it is active 

 and voracious — but usually falls into a kind of torpor, during which 

 the changes take place which issue in the flying insect ; during the 

 passive stage of metamorphosis it is called a * pupa ' ; the last volant 

 stage is the ' imago.' 



The chief steps in the metamorphosis were traced as they affect the 

 outward form, the digestive organs, the circulatory, and respiratory, 

 and nervous systems. 



The main differences in the metamorphoses of insects relate to the 

 place where, and the time during which they are undergone. The 

 young cockroach and the little aphis, which were first acephalous 

 and apodal, and then had thirteen equal segments, with soft un- 

 jointed legs, proceed to acquire a distinct head with antennae, a 

 thorax with three pairs of long jointed legs, and an abdomen, before 

 they quit the egg ; they thus enter upon active life under the guise 

 of a crab, instead of a worm. With regard to the Aphis, that insect, 

 instead of proceeding to perfect its individual development, may at 

 once begin the great business of its existence by parthenogenetic 

 procreation. Bonnet's experiments, which first brought to light this 

 marvellous fact, have received uniform confirmation from all subse- 

 quent inquirers, and no natural phsenomenon is now better deter- 

 mined. 



From seven to eleven successive generations have been traced 

 before the individual has finally metamorphosed itself into the 

 winged male or winged oviparous female. 



In autumn, when the nights grow chilly and long, the oviparous 

 imago completes her duty by depositing the eggs in the axils of the 

 leaves of the plant, where they are protected from the winter frost, 

 and ready to be hatched at the return of spring. Then recom- 

 mences the cycle of change, which being carried through a succes- 

 sion of individuals and not completed in a single life-time, is a 

 ' metagenesis ' rather than a • metamorphosis.' 



This phsenomenon, which until very recently was deemed an 

 exception, and a most marvellous one, in Nature, now proves to be 

 an example of a condition of procreation to which the greater part 

 of organized Nature is subject. 



The Lecturer was inevitably limited in his choice of illustrations : 

 and proceeded to an instance of metagenesis from the radiated sub- 

 kingdom of animals. 



The stages of this metagenesis have been best and most completely 

 traced in the Medusa aurita, by Siebold, Daly ell, Sars, and others. 



