THE PRINCIPLES OF BIOLOGY. 
127 
each more obvious individual is composed of twenty, or fewer, 
or, it may be, of very many more such less-independent though 
highly complex individuals, these being more or less perfectly 
fused into the greater unity. And probably no student of 
these forms has ever comprehended the structure of an 
earthworm, without asking himself if the jointed chain of his 
own backbone does not point to a like doubly-compound 
origin of his body, in common with those of all other 
vertebrates. This interesting question of “ Morphological 
Composition” Mr. Spencer has considered in the early part 
of this volume, and the chapter on the vertebrate skeleton, 
as well as the appendix relating to the same subject, are 
practically continuations of that discussion. 
To the great English osteologist—Sir Richard Owen—as 
to the famous Goethe and others, this theory of the serial 
origin of every vertebrate animal, commended itself as being 
indicated by the long succession of somewhat similar bones, 
and sets of bones, which constitute the principal portion of 
their skeletons. He taught that each member of this worm- 
like chain of segments is but a modification of one “archetypal” 
pattern, from which all the many existing varieties have been 
derived. The classical works of Sir Richard are dominated bv 
this conception ; and on this ground Mr. Spencer bases an 
adverse criticism.* At the same time he pays a willing 
tribute to the unrivalled knowledge, and great abilities of this 
most celebrated of osteologists, and acknowledges his own 
indebtedness both to his writings and lectures. 
In opposition to this theory of the multiplex origin of the 
vertebrates, Mr. Spencer advances his own view, that the 
jointing of the backbone is not a remnant of an original com¬ 
pound individuality ; but that it is due to later changes, which 
have been produced in the originally continuous cartilaginous 
notochord, as by ossification it became too rigid to admit of 
the necessary flexures without jointing. 
The statement of this latter theory is prefaced by a dis¬ 
cussion of the distribution of the strains in a beam. First, 
when it is depressed in the ordinary way by a load acting in 
one direction only ; secondly, when the bending force acts 
alternately in opposite directions. It is shown that by these 
alternate bendings both the upper and lower parts of the beam 
are alike subject first to compression and then to tension ; while 
there is a portion along the middle of the beam which is 
never stretched, but always compressed, let the bending come 
from which side it may. 
* “ Principles of Biology,” Yol. II., Appendix B. 
