LAWS OF ORGANIC DEVELOPMENT. 599 
epitomized by Spencer as the ‘‘ preservation of the fittest.” This 
neat expression no doubt covers the case, but it leaves the origin 
of the fittest entirely untouched. Darwin assumes a ‘‘ tendency to 
variation” in nature, and it is plainly necessary to do this in order 
that materials for the exercise of a selection should exist. Dar- 
win and Wallace’s law is, then, only restrictive, directive, conser- 
vative, or destructive of something already created. Let us, then, 
seek for the originative laws by which these subjects are furnished 
—in other words, for the causes of the origin of the fittest. 
The origin of new structures which distinguish one generation 
from those which have preceded it, I have stated to take place 
under the law of acceleration. As growth (creation) of parts 
usually ceases with maturity, it is entirely plain that the process 
of acceleration is limited to the period of infancy and youth in all 
nimals. It is also plain that the question of growth is one of 
nutrition, or of the construction of organs and tissues out of pro- 
toplasm. 
The sotstsadtion of the animal types is restricted to two kinds 
of increase—the addition of identical segments and the addition 
of identical cells. The first is probably to be referred to the last, 
but the laws which give rise to it cannot be here explained. Cer- 
tain it is that segmentation is not only produced by addition of 
identical parts, but also by subdivision of a homogeneous part. 
In reducing the vertebrate or most complex animal to its simplest 
expression, we find that all its specialized parts are but modifica- 
tions of the segment, either simply or as sub-segments of com- 
pound but identical segments. Gegenbaur has pointed out that 
the most complex limb with hand or foot is constructed, first, of 
a single longitudinal series of identical segments, from each of 
which a similar segment diverges, the whole forming parallel series, 
not only in the oblique transverse, but generally in the longitudi- 
nal sense. Thus the limb of the Lepidosiren represents the simple 
type, that of the Icthyosaurus a first modification. In the latter 
the first segment only (femur or humerus) is specialized, the other 
pieces being undistinguishable. In the Plesiosaurian paddle the 
separate parts are distinguished ; the ulna and radius well marked, 
the carpal pieces hexagonal, the phalanges well marked, etc. 
As regards the whole skeleton the same position may be sadely 
assumed. Though Huxley may reject Owen’s theory of the verte- 
brate character of the segments of the cranium, because they are 
