154 Nerve Structure and Force. [April, 



and then come excretory organs, such as the kidneys, to deprive the 

 blood of all noxious principles. But in the invertebrata, the mode of 

 alimentation is often more simple, although we may find the animal 

 supplied with other organs fitted for its own preservation, but not 

 directly for propagation, such as poison-bags, spinning apparatus of 

 spiders, the ink-bag of the cephalopods. 



Under most conditions in the animal world the act of repro- 

 duction takes place by means of an ovum or egg, which is fecundated 

 by the spermatozoon, a mysterious germ moving about in a peculiar 

 secretion. The organs, therefore, subservient to such functions are 

 the most important. But other modes consist in the act of spon- 

 taneous fission or division of the parent animal, or by the act of 

 budding or gemmation. 



The young animal which leaves the egg is very often an exact 

 counterpart of the parent, differing only in size, as in the case of 

 the common chick ; but in a vast number of instances this resem- 

 blance does not exist ; sometimes the young require yet further evo- 

 lution and change ; in other cases there is no resemblance at all, 

 either in form or mode of living ; and this brood may repeat itself in 

 its dissimilar character, or may again acquire the peculiar organs which 

 bring it back to the parent shape. Then again we have the well- 

 known metamorphoses as illustrated by the larvae of insects, in which 

 the offspring, though unlike the mother, passes through a definite 

 series of changes, until it again acquires the parent form, again to 

 reproduce in like fashion, and finally to die. 



In all animals this function seems under the direction of what, 

 for want of a better term, has been called " vital principle," " nerve 

 power," &c. — not that brain ganglia or nerve fibres can invariably be 

 found — but the changes are such as can hardly be explained on the 

 ground of mere chemical affinity or of nutrition and assimilation. 



What have we to say of nerve-power? Viewing it from its 

 lowest manifestation, we may regard it as some dynamic force, 

 evolved and regulated by co-existing, but yet unknown laws. 

 Regarded from its higher attributes, we must incline to a principle 

 of life, which is something superadded to ordinary laws, something 

 which, while it lasts, keeps in abeyance the usual elements of dete- 

 rioration and decay. Call it what we will, the i>vyj t ; the vital 

 principle ; its presence expresses life and creative or constructive 

 action ; its absence, decay, dissolution, degradation. 



In the lowest forms of animal life, its presence is recognized by 

 no known structure, but we soon observe nerve ganglia and fibres. 

 They first appear about the oral aperture, their fibres extend to the 

 radiating tentacles. The distribution of such nerve fibres and gan- 

 glia depends on the form of the individual, so that in myriapoda and 

 annelida, they are repeated at segmental intervals, or concentrated 

 in insecta, where the vitality is most clearly manifested in the head. 



