Bateson.[420]

Bateson broke completely with the Dohrn-Semper view that the metamerism
of Articulates and Vertebrates must be put down to inheritance from a
common ancestor. He held that metamerism was merely a special
manifestation of the general property of repetition, common to all
living things (_cf._ Owen's "vegetative force"), and that accordingly
"however far back a segmented ancestor of a segmented descendant may
possibly be found, yet ultimately the form has still to be sought for in
which these repetitions had their origin" (p. 549). The meaning of the
phenomenon was obscure, but he was convinced that the explanation was
not to be found in ancestry. "This much alone is clear," he wrote, "that
the meaning of cases of complex repetition will not be found in the
search for an ancestral form, which, itself presenting this same
character, may be twisted into a representation of its supposed
descendant. Such forms there may be, but in finding them the real
problem is not even resolved a single stage; for from whence was their
repetition derived? The answer to this question can only come in a
fuller understanding of the laws of growth and of variation, which are
as yet merely terms" (pp. 548-9). It was in following up this line of
thought that Bateson produced his monumental _Materials for the Study of
Variation_ (1894).

He found a strong positive argument for his theory that Vertebrates are
descended from unsegmented forms in the fact that the notochord arises
as an unsegmented structure. With the notochord he homologised the
supporting rod in the proboscis of _Balanoglossus_, which like the
notochord arises from the dorsal wall of the archenteron, and has a
vacuolated structure. The gill-slits of _Balanoglossus_, with their
close resemblance in detail to those of Amphioxus, Bateson also used as
an argument in favour of the phylogenetic relationship of the
Enteropneusta and Vertebrata, together with the formation from the
ectoderm of a dorsal nerve tube.

Bateson's views attracted considerable attention, and were thought by
many to lighten appreciably the obscurity in which the origin of
Vertebrates was wrapped. Thus Lankester wrote in his article on
Vertebrates[421] in the _Encyclopedia Britannica_:--"It seems that in
_Balanoglossus_ we at last find a form which, though no doubt
specialised for its burrowing sand-life, and possibly to some extent
degenerate, yet has not to any large extent fallen from an ancest