28 Mr. H. G. Seeley on the Origin 



hollowness of the limb-bones, the ilia, &c., potentially (per- 

 haps) in the formation of investing epiphyses at their ends — 

 but in scarcely an appreciable way upon the vertebrae in either 

 form, since they remain both very few in number and short. 



It cannot be necessary to multiply these illustrations ; for 

 the same law may be traced in every osseous structure. Where 

 an animal uses any part of the body, the part grows long, 

 either kinetically by lengthening the individual parts, or 

 potentially by increasing their number. 



If, now, we generalize these facts in relation to the vertebral 

 column, the result is, that since the potential epiphyses multi- 

 tiply indefinitely and elongate the body, so there must have 

 been a period when the body was short and when the seg- 

 ments were very few — and that the elongation of the body 

 proceeds gradually, and, except in the caudal region, is likely 

 to be arrested by the development of limbs. 



It were simplest to assume, if there had been grounds for 

 doing so, a single vertebra as the basis from which the body 

 was formed ; but the existence of a notochord among tunica- 

 ries, and the vast gap between Amphioxus and ordinary 

 vertebrates, does not warrant such an assumption ; nor does it 

 indeed enter practically into my theory of a vertebrate. How- 

 ever, if we assume an animal with the viscera of a fish, with 

 a notochord, and with terminal muscles capable of moving the 

 tail, then the consequence of that arrangement would be the 

 formation of a terminal segment, not by breaking a piece off 

 the notochord, but by the muscular action increasing the 

 density of the terminal portion, and this organic dialysis even- 

 tually giving it a structure by which it is chemically separated 

 from the other parts. The direction of the mechanical strain 

 becomes the direction of greatest density, and determines the 

 directions in which the osseous matter is deposited and the 

 shapes Avhich it assumes. 



Then, just as the inherited energy of many individuals at 

 last became a force sufficient to differentiate the first osseous 

 caudal segment, so the continuous operation of the same mus- 

 cles goes on accumulating energy for which there can be no 

 outlet in the adult organization, and the energy takes the 

 potential form. It has, in fact, become so powerful that, in- 

 stead of displaying itself only in maturity, it begins to act 

 upon the immature animal at as early a time as the other and 

 .ordinary laws of its growth, and in this way gives expression 

 to itself, differentiating a new segment similar to the pre- 

 existing segment — a potential epiphysis, which, growing con- 

 tinuously with the original segment, can afterwards scarcely 

 be distinguished from it. Thus the tail comes to have two 



