of the Vertebrate Skeleton. 23 



body experiences a serpentine motion originated by the tail 

 and passing forward. To understand clearly the effects upon 

 the animal of this movement, it will be useful to study it ex- 

 perimentally. If, then, I take an ordinary long bolster, which 

 in its cylindrical form will represent a fish, and hold firmly 

 one extremity of it (which for convenience I suppose to be its 

 tail), and then imitate the movement of the fish by moving 

 the tail powerfully from side to side, it will be seen that the 

 movement propels the feathers towards the free end of the 

 bolster ; that is, by granting the bolster a tail, I have elabo- 

 rated for it a head also. Now to apjoly this principle to the fish. 

 Instead of the force furnished by my hands, there are enor- 

 mous muscles extending down the body ; instead of the bed- 

 ticking for an outer envelope, there is a vertebral column ; 

 and finally, instead of feathers inside of it, there is the central 

 nervous system, which, in the young state at least, is centrally 

 fluid. Now, if the tail is set moving as it is seen to move in 

 a fish out of water, the powerful pressure behind will compress 

 the light semifluid substance of the spinal cord and force it to 

 move forward, and this movement is maintained during the 

 life of the individual ; it will also by the tension increase the 

 lengtli of the spinal matter relatively to the osseous sheath. 

 The mechanical effect, then, of motion originated by the tail 

 is an immense amount of leverage applied at every point of 

 the curve of the body, which inevitably acts upon the con- 

 tents of the spinal tube in compressing and forcing the sub- 

 stance forward. It also must act, as all tension and pressm'e 

 have been seen to act, in stimulating the growth of the spinal 

 cord. 



Thus there is a persistent influence ever tending to elongate 

 the spinal column. As it was seen that there is an actual 

 forcing of the spinal cord forward, so this growth will tend in 

 the same way. But I have already pointed out how soon the 

 individual power to be modified in form comes to an end,, 

 although the forces capable of modifying the organism con- 

 tinue to act, — and that thus the energy of life is not lost, but 

 becomes potential for a time in the parent, and can only be 

 manifested kinetically when a bud or ovum which has in it a 

 capacity for mobility which the parent had not, is thrown off 

 from the organism ; and then, under the name of a variety, 

 we see manifested the potential activity of the parent which 

 its organization had previously compelled to remain as poten- 

 tial activity. So that we cannot expect to find these forces 

 producing large visible effects under our eyes in one indivi- 

 dual. But we must expect that in a succession of individuals, 

 each of which remains for a certain period capable of modifi- 



