36 THE CAUSES WHICH PROPAGATE 



these and other propensities wliich operate as stimulants in the 

 system itself^, the naturalist has found that lig-ht, heat, and 

 moisture, in various degrees, from absolute darkness, coldness, 

 and dryness, act as stimulants upon living bodies ; he has 

 experienced that electricity is a general agent, that several 

 plants emit flashes, and that some animals even give shocks 

 resembling the electric, while galvanism produces, and, as many 

 consider, originates, both muscular and nervous action. 



Nothing exercises a more despotic power over the fortunes 

 of lower life than habit and instinct. Witness in proof of this 

 statement the perfection and unerring nature of the acts and 

 duties in which the bee, wasp, or ant engages I'rom the first 

 moment of existence. The very excellence of the acts performed 

 by these unreasoning creatures is, as Dr. Carpenter has remarked, 

 a proof of a non-intelligent nature. Insects likewise from 

 organisation, one would infer, must be esjoecially a prey of 

 propensity or instinct, since the double ventral chore! receiving 

 the several ramifications of their nervous system at the rings of 

 the body, in a series of knots or ganglions, has its first swelling, 

 to which we commonly assign the name and notion of brain, 

 either little different or so much smaller than the rest, which 

 equally receive their groups of nerve branches, that we might, 

 in a majority of cases, suppose it rather formed to be guided by 

 their indications than suited to control them. The reader will 

 readily understand this by referring to Plate VII., where the 

 nervous systems of various insects are sketched out. Then as the 

 insect grows and develops to its last adult condition, the knots or 

 ganglions take corresponding change. In the moth (Fig. 3) we 

 see the first in order are larger than those of the caterpillar 

 (Fig. 5) ; they are, in cases where they are also marked in the 

 larvae, commonly less numerous, and, in the jM-esent instance, 

 two of the larger appear to have approximated. Among insects 

 in their final perfect condition, this singularity is best seen 

 in the nervous system of the Cicada shown at Fig. 1. But with 

 this change in the nerve-knots, change in the habit and instinct 

 may be associated ; and in the pronounced case of the crawling 

 caterpillar and flying moth, an almost complete revolution is 

 perceptible. An eminent French zoologist. Dr. Virey, has 

 compaied the animal in this case to a hand organ, in which, on 



