August 9, 1877] 



NATURE 



289 



EVOLUTION OF NERVES AND NERVO- 

 SYSTEMS^ 



III. 



THE question, however, remains : Will this conductile 

 function prove itself as tolerant towards section of the 

 tissue as the contractile function has already proved itself 

 to be ? for, if so, any objection to the view that the passage 

 of the contractile waves is due to vicarious action of 

 rudimentary nerve-fibres will be removed. Briefly, the 

 answer to this question is an affirmative ; for I find it is 

 quite as difficult to block the passage of stimulus waves 

 by means of interposing cuts, as we have seen that it is to 

 block the passage of contractile waves by the same means. 

 For instance, here is an Aurelia (Fig. 5), the bell of which 

 has been cut into the form of a contmuous parallelogram 

 of tissue, and then submitted to the tremendously severe 

 form of section which is depicted. Yet on very gently 

 stimulating any point in this expanse of tissue, as at the 

 end a, a tentacular wave would course all the way along the 

 margin, to b, thus showing that the wave of stimulation 

 must have passed round and round the ends of all the 

 intervening cuts. In the diagram the tentacular wave is 

 represented as having traversed one-half of the whole 

 distance from a to b, and near b there is represented a 

 single remaining ganglion, {g). When, therefore, the 

 tentacular wave reaches ^t,', this ganglion will shortly after- 

 wards discharge, so givmg rise to a contractile wave, 

 which will course back from g to a in the opposite direction 

 to that which the stimulus wave had previously pursued. 



And this, I am not afraid to say, is the most important 

 observation, both to the physiologist and to the evolu- 

 tionist, that has ever been made in the whole range of 

 invertebrate physiology. For to the physiologist this 

 observation proves that the distinguishing function of 

 nerve, where it first appears upon the scene of life, is a 

 function which admits of being performed vicariously, to 

 almost any extent, by all parts of the same tissue-mass ; 

 while to the evolutionist the observation proves the exist- 

 ence of such a state of things as his theory of nervo- 

 genesis would lead him to expect. In such a symmetri- 

 cally-formed animal as a Medusa, with all parts of the 

 contractile sheet precisely resembhng one another, we 

 should expect the lines of discharge composing the hypo- 

 thetical plexus to be very numerous, and all very much 

 alike with respect to the degree of their evolution. For, 

 as the symmetrical form of the disk does not require that 

 any one set of lines should be used much more frequently 

 than any other set, it follows from Mr. Spencer's theory 

 that all the Hnes should more or less resemble one 

 another as regards the extent of their differentiation.^ 

 That is to say, they should all be lines presenting about 

 the same degree of resistance to the passage of a stimulus 

 wave, and therefore it should become a matter of indif- 

 ference, so to speak, through which particular set of lines 

 such a wave takes its course. 



There is still another class of facts which to my mind 

 makes very strongly in favour of Mr. Spencer's theory. 



delivered at the Royal Institution on Friday 

 By George J. Romanes, iM.A., F.L.S., &c. 



' Abstract of a Lectu 

 evening. May 25, 1877. 

 Continued from p. 27 i. 



2 Mr. Spencer himself observes, "The average equality of the forces to 

 which their bodies (i-i'., those of the Medusae) are exposed all round is un- 

 favourable to the formation of distinct muscles and a distinct nervous 

 sysfm " (" Psychology," vr.l. i. p. 522) Although this statement must now be 

 modified so far as the ganglionic system of the Medusse is concerned, I do 

 not think that the anticipation which it embodies should on this account be 

 deemed unwarrantable so far as it applies to other parts of the nervous system. 

 For although it is true that a Medusa as a tulwte is " exposed all round " to an 

 *■ average equality of forces," it is not true that the excitabte portions of a 

 Medusa are thus equally exposed. On th*^ contrary, the margin of the 

 excitable sheet which lines the cavity of the bell, occupies a much more 

 exposed posiiion than does any other part of that sheet : and whether or not 

 this fact has anything to do with the development of the ganglia in the only 

 part of the excitable sheet which is thus peculiarly situated, I think it is 

 obvious that this part of a Medusa ought to be carefully excepted in the 

 statement which 1 have quoted. With regard to all other paits of the 

 excitable sheet, however, the statement is certainly correct ; and it is only to 

 such parts that the considerations in the text apply — G. J. R. 



Assuming, as I think we are now entitled to assume, that 

 the contractile waves are not merely muscle waves, but 

 depend for their passage on the progressive passage of 

 the stimulus waves — assuming this, the following facts 

 become facts of great significance. When the con- 

 tractile waves in a spiral strip have become suddenly 

 blocked by section, in the great majority of cases, 

 such blocking will be permanent — even though the strip 

 be continously stimulated, whether artificially or by a 

 single terminal ganglion, as represented in Fig. 4. But 

 in the remaining cases, after a time that varies from 

 a few minutes to a day or more, the obstruction is 

 overcome, and the contractile waves pass forward with 

 perfect freedom. Now, if I had time, I could prove that 

 these facts are certainly not to be attributed to what 

 physiologists term shock; and, therefore, it seems to 

 me that only one hypothesis remains. What I have 

 recently said about most of the lines of discharge in the 

 supposed plexus being very much alike as regards the 

 degree of their differentiation, does not, of course, mean 

 that all the lines are exactly alike in this respect ; for on 

 J /r/or/ grounds such a state of things would be in the 

 last degree improbable. Consequently, in conducting a 

 spiral section, it must happen that at every snip the 

 scissors cut through a number of lines of discharge pre- 

 senting various degrees of differentiation ; and, such 

 being the case, the fact of the sudden and final blocking 

 is presumably due to a well-differentiated line having been 

 severed in a part of the tissue where no other Ime occurs 

 of a sufficient degree of differentiation to conduct the 

 stimulus forward. Now in most instances, as we should 

 expect, the blocking so caused is permanent ; for it is 

 manifest that th° formation of nervous channels, in the 

 way suggested by Mr. Spencer, cannot proceed at so great 

 a rate as to admit of ivlwlly new lines of discharge being 

 established during the life-time of a mutilated Medusa, i.e., 

 during the course of a few days. Nevertheless, according 

 to the hypothesis, some small percentage of cases might 

 be expected to occur in which such blocking of the contrac- 

 tile waves would only be temporary. For some cases would 

 almost certainly occur in which the relations of the highly 

 differentiated line just destroyed to the more slightly 

 differentiated lines in the neighbourhood of the section, 

 would happen to be such that the more slightly dif- 

 ferentiated lines would be very nearly, though not quite, 

 able to act vicariously for the more highly differentiated 

 line which has just been destroyed (see Fig. 4, where 

 the deep line represents the well-differentiated line which 

 has just been severed, and the dotted line the less-differ- 

 entiated one which is still intact). The contractile waves, 

 therefore, would in the first instance become suddenly 

 blocked at the end of the strip. But the molecular, and 

 with them the contractile, waves still continuing to pass 

 quite up to the end of the strip, and being there always 

 suddenly stopped, a rude conflict of molecular forces will 

 thus set up in the area where these waves are impeded, 

 and each of the forces concerned will seek for itself the line 

 of least resistance. Hence, as the successive waves beat 

 rhythmically on the area of obstruction, more or less 

 of the molecular disturbance must every time be equalised 

 through those lines of discharge which from the first have 

 been almost sufficient to maintain the physiological con- 

 tinuity of the tissue. Therefore, according to the hypo- 

 thesis, every wave that is blocked imposes on these 

 particular lines of discharge a much higher degree of 

 functional activity than they were ever before required to 

 exercise ; and this greater activity causing in its turn 

 greater permeability, a point will sooner or later arrive at 

 which these lines of discharge from having been almost 

 become quite able to draft off sufficient molecular motion, 

 or stimulating influence, to carry on the contractile waves 

 beyond the area of previous blocking. In such instances, 

 of course, we should expect to find, what I always observed 

 to be the case, viz., that the first contractile waves which 



