128 
THE PRINCIPLES OF BIOLOGY. 
The case of a fish with its alternate bendings from side to 
side, as it propels itself through the water, is seen to present a 
sufficiently parallel case. In the animal’s body, as in the 
beam, a central part is compressed by every movement; and 
this part is advantageously composed of a comparatively 
unyielding substance. In older types this appears as the 
notochord, rigid enough to resist the compressions which 
occur, while retaining a considerable amount of flexibility. 
But as size, or activity, or both increase, a more resistent 
fulcrum is needed ; and the deposition of bone becomes 
gradually manifest, both in passing from lower to higher 
forms, and also in the course of the embryological develop¬ 
ment of the latter. 
To continue in Mr. Spencer’s own words (p. 204): “ If, as 
facts warrant us in supposing, a formation of denser substance 
occurs at those parts of the notochord where the strain is 
greatest, it is clear that this formation cannot so go on as to 
produce a continuous mass ; the perpetual flexions must 
prevent this. If matter that will not yield at each bend is 
deposited while the bendings are continually taking place, 
the bendings will maintain certain places of discontinuity in 
the deposit—places at which the whole of the stretching 
consequent on each bend will be concentrated. And thus 
the tendency will be to form segments of hard tissue capable 
of great resistance to compression, with intervals filled by 
elastic tissue capable of great resistance to extension'—a 
vertebral column.” 
And in conclusion we read again (p. 208):—“ Of course, 
the foregoing synthesis is to be taken simply as an adumbra¬ 
tion of the process by which the vertebrate structure may 
have arisen through the continued actions of known agencies. 
The motive for attempting it has been twofold. Having, as 
before said, given reasons for concluding that the segments 
of a vertebrate animal are not homologous in the same sense 
as those of an annulose animal, or a pluenogamic axis, it 
seemed desirable to do something towards showing how they 
are otherwise to be accounted for. . . .” 
“ Leaving out all that is hypothetical, the general argument 
maybe briefly presented thus:—The evolutionfromtlie simplest 
known vertebrate animal, of a powerful and active vertebrate 
animal, implies the development of a stronger fulcrum. The 
internal fulcrum cannot be made stronger without becoming 
more dense. And it cannot become more dense while retain¬ 
ing its lateral flexibility without becoming divided into 
segments. Further, in conformity with the general principles 
thus far traced, these segments must be alike in proportion 
