82 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 a priori likely 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. 



