550 WILLIAM BATESON, 
is no doubt true, but these correspondences are only partial, 
and, as will be shown in the sections on the nervous system 
and vertebral column, a history is preserved to us of the steps 
by which some, at least, of these repetitions have been attained 
and of stages in which these correspondences were still more 
irregular. 
The attempt to find the ancestor of the Chordata resolves 
itself first into the question as to whether the Chordate features, 
viz. notochord, gill-slits, and nervous system of a particular 
type were first associated in a form which possessed repetitions 
in a high degree or not. Now, since the notochord is always 
unsegmented, it is 4 priori hkely that it arose in an unseg- 
mented form; for, having in view the early period of develop- 
ment at which it arises and the situation which it occupies in 
the body, and the fact that it is found in the dorsal wall of the 
gut, the sacculation of which is one of the commonest features 
in segmented forms, it could hardly have thus arisen without 
participation in such segmention. On the hypothesis of 
Annelid descent the facts of the morphology of the notochord 
are inexplicable; for, seeing that no homologue of the noto- 
chord exists among Annelids, on the theory that Vertebrates 
are their descendants, the notochord must have arisen sub- 
sequently to that segmentation, to account for which the 
Annelid ancestor is postulated. If this were so the notochord, 
by every rule of phylogenetic interpretation, might be expected 
to arise late in development, and to exhibit marked segmenta- 
tion, instead of which it is almost the earliest organ formed, 
and is absolutely unsegmented. 
Similarly from the first, the medullary plate is distinctly a 
single structure, and without suggestion of transverse division. 
Not until the peripheral nerves arise is any serial repetition to 
be found in it, and were it not for theoretical considerations it 
would not have been supposed that the nervous system of a 
two-day Chick was a segmented structure. Further, in Am- 
phioxus and the Marsipobranchs the serial repetition, even of 
the peripheral nerves, is not regular and opposite, the further 
meaning of which facts will be discussed later. 
