1871.] 71313) 
[Cope- 
gree of this modification. This retardation is also of systematic importance, 
and, should either of the characters described be constant in any of the 
species of the genus Homo, would at once entitle it to new generic rank. 
The very frequent absence of the posterior molars (wisdom teeth) has 
been recently found to characterize a race in India. Should this peculi- 
arity prove constant, this race would with propriety be referred to as a 
new genus of Hominidx, as we have many cases of very similar species 
being referred to different genera. It is altogether probable that such 
will, at some future time be the condition of some race or races of men.* 
Iam now disposed to regard the above as the method of production, 
not only of generic but of all other, including specific characters. It 
would appear that by excessive acceleration or retardation, some of the 
characters of a series may be skipped, but observations are not conclusive 
on this point, since very close examination is necessary for the apprecia- 
tion ef very transitory embryonic conditions. 
II. ON THE LAW OF REPETITIVE ADDITION. 
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 animals. It is also plain that the question of 
growth is one of nutrition, or of the construction of organs and tissues 
out of protoplasm. 
The construction of the animal types may be referred 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 now be explained. Certain 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 ani- 
mal to its simplest expression, we find that all its specialized parts are 
but modifications of the segment, either simply or as sub-segments of 
compound but identical segments. Gegenbaur has pointed out that the 
most complex limb with hand or foot, is construc ted, 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 ob- 
lique transverse, but generally in the longitudinal sense. Thus, the limb 
of the Lepidosiren represents the simple type, that of the Ichthyosaurus a 
modification. In the latter, the first segment only (femur or humerus) is 
specialized, the other pieces being undistinguishable. In the Plesiosaur- 
ian paddle the separate parts are distinguished; the ulna and radius well 
marked, the carpal pieces hexagonal, the phalanges defined, etc. 
As regards the whole skeleton the same position may be safely as- 
sumed. Though Huxley may reject Owen’s theory of the vetebrate char- 
*The preceding section is merely an abbreviation with new illustrations, of the pro 
positions brought forward in the writers “ Origin of Genera,” 1868, where a considera- 
ble extension of the subject will be found. 
