52 THE POPULAR SCIENCE MONTHLY. 



must, as was formerly supposed, have been directly impressed upon the- 

 nervous oi'gauization at the creation of the tirst bee, and transmitted 

 by each succeeding generation, or, as contended by Herbert Spencer 

 and others, the race must have become gradually endowed Avith it, by 

 a constant repetition of those acts which each individual was stimu- 

 lated to perform by its surroundings at succeeding times. The former 

 method presupposes a special creation and endowment for each species 

 of animals ; a supposition generally rejected by scientitic men as pre- 

 senting insurmountable ditiiculties, and as not having facts within pos- 

 sible reach to sustain it ; for no one has ever known of a special crea- 

 tion. The other method presuj^poses development in some of its vari- 

 ous phases, which, although not Avithout its difficulties, satisfies so 

 many existing conditions, and is constantly helping to solve so many 

 formerly insoluble problems, that scientific men are led to adopt it, 

 provisionally at least, as probably true in its main features, and cer- 

 tainly of great importance as an aid in further investigations. 



According to this theory, instinct is the aggregate or accumulated 

 experience of each race of creatures in which it is found is impressed 

 by repetition upon the nervous organization, and is inherited alike by 

 each individual of a race, causing their actions to be the same, genera- 

 tion after generation, unless changed by necessity from changed sur- 

 roundings. This is the mode of action characterizing the large class 

 of animals whose highest nervous develojiment is the sensorium. It 

 embraces the cephalous moUusks and the whole division of articulates;, 

 and its highest development is reached in insects. 



Ascending now another step in the series of animated forms, we 

 find again an advance in the nervous organization suited to the still 

 more complex action and consequent needs of the animal. 



Without following out in detail the gradations which take place in 

 passing from the articulate to the vertebrate series, we find in general 

 terms the following changes to have occurred ; and, as an example,, 

 one of the first and simplest of the series may be taken, namely, the 

 fish. Instead of the ventral cord, with its interrupted series of gan- 

 glia, as in the centipede, we find in the fish a spincd cord, existing as a 

 continuous line of ganglionic matter, inclosed in a fibrous sheath of 

 white conducting matter, and the whole protected in the bony canal 

 formed for it by the beautifully-arranged pieces which make uji the 

 vertebral column, or back-bone. From this continuous ganglionic 

 centre nerves are given ofl" at different points, as they are needed to- 

 supply the diflerent muscles of the body, and esj^ecially those of loco- 

 motion. "We find the sensorium enlarged to preside over the im- 

 proved organs of special sense, the optic ganglia still being much the 

 more important ; and we have two 7teic ganglionic masses added, the 

 cerebnan in front of the optic ganglia, and the cerebellum, placed just 

 behind them. Concerning these, it may be remarked that the latter, 

 although its function has not been so clearly demonstrated as many 



