OF LIFE, AND ITS CONDITIONS. 33 



12. We are now prepared to inquire into the raanifestions of Vital Ac- 

 tivity in the Animal, and into the sources of the power by which the various 

 form's of that activity are sustained. The first of the manifestations is, as 

 in the Plant, the building up of the organism by the appropriation of ma- 

 terial supplied from external sources under the directive agency of the germ. 

 The ovum of the Animal, like the seed of the Plant, contains a store of ap- 

 propriate nutriment previously elaborated by the parent ; and this store suf- 

 fices for the development of the embryo, up to the period at which it can ob- 

 tain and digest alimentary materials for itself. That period occurs in the 

 different tribes of animals at very dissimilar stages of the entire develop- 

 mental process. In many of the lower classes, the embryo comes forth from 

 the egg, and commences its independent existence, in a condition which, as 

 compared with the adult form, would be as if a Human embryo were thrown 

 upon the world to obtain its own subsistence only a few weeks after concep- 

 tion ; and its whole subsequent growth and development take place at the 

 expense of the nutriment which the embryo ingests for itself. We have ex- 

 amples of this in the class of Insects, many of which come forth from the 

 egg in the state of extremely simple and minute worms, having scarcely any 

 power of movement, but an extraordinary voracity. The eggs having been 

 deposited in situations fitted to afford an ample supply of appropriate nutri- 

 ment (those of the flesh-fly, for example, being laid in carcases, and those of 

 the cabbage butterfly upon cabbage leaves), each larva on its emersion is as 

 well provided with alimentary material as if it had been furnished with a 

 large supplemental yolk of its own ; and by availing itself of this, it speedily 

 grows to many hundred or even many thousand times its original size, with- 

 out making any considerable advance in development. But having thus 

 laid up in its tissues a large additional store of material, it passes into a 

 state which, so far as the external manifestations of life are concerned, is one 

 of torpor, but which is really one of great developmental activity; for it is 

 during the pupa state that those new parts are evolved, which are character- 

 istic of the perfect Insect, and of which scarcely a trace was discoverable 

 in the larva; so that the assumption of this state may lv likened in many 

 respects to a re-entrance of the larva into the ovum. On its termination, 

 the Imago or perfect Insect comes forth complete in all its parts, and soon 

 manifests the locomotive and sensorial powers by which it is specially dis- 

 tinguished, and of which the extraordinary predominance seems to justify 

 our regarding Insects as the types of purely Animal life. There are some 

 Insects whose Imago-life has but a very short duration, the performance of 

 the generative act being apparently the only object of this stage of their ex- 

 istence : and such for the most part take no food whatever after their final 

 emersion, their vital activity being maintained, for the short period it en- 

 dures, by the material assimilated during their larva state. 1 But those whose 

 period of activity is prolonged, and upon whose energy there are extraordi- 

 nary demands, are scarcely less voracious in their Imago than in their Larva- 

 condition ; the food they consume not being applied to the increase of their 

 bodies, which grow very little after the assumption of the Imago-state, but 

 chiefly to their maintenance, no inconsiderable portion of it, however, being 

 appropriated in the female to the production of ova, the entire mass of which 

 deposited by a single individual is sometimes enormous. That the perform- 

 ance of the generative act involves not merely a consumption of Material, 



1 It is not a little curious that in the tribe of Rntifera, or Wheel-animalcules, all 

 the males yet discovered are entirely nxplmtchnic: not only the whole of their develop- 

 ment within the egg. but the whole of their active life after their emersion from it, 

 being carried on at the expense of the store of yolk provided by the parent. 



