556 WILLIAM BATESON. 
been used to throw light on the origin of the group, and they 
had to be expressly excluded because the suggestion as to the 
origin of the group had been made without regard to them. 
In the case of Amphioxus and the Marsipobranchs this theory 
of degeneracy will not bear examination. 
It rests solely in the one case on the fact that Amphioxus 
has no developed sense organs and lives buried in the sand, and 
in the other on the semi-parasitic habit of life of the group. 
This degeneration is postulated to explain the lower degree of 
segmentation presented by these forms; and the fact remains 
that of all animals the worms which live most underground are 
the most segmented types which are known. Hence it cannot 
be assumed without ontogenetic evidence that degeneration in 
this direction has occurred. This ontogenetic evidence is en- 
tirely absent. Degeneration in this sense means a phylo- 
genetic change of plan; and this change of plan should then 
leave a mark on the ontogeny, as occurs in Kchiurus, &c.; but no 
event in the development of Amphioxus or of Lampreys points 
to any such change of plan. The development of these forms 
is a steady progress up to the point which the creatures finally 
reach, and in a case of this kind it is gratuitous to postulate 
degeneration in order to support a preconceived view of the 
morphology of the group. (Even in the Ascidians, though a 
well-marked change of this kind does occur, yet it is not a 
deviation from a segmented to a less segmented form ; for with 
the doubtful exception of Appendicularia, Ascidian tadpoles 
are quite without trace of segmentation.) 
Again, no such evidence of a change of phylogenetic plan is 
found in the case of the Enteropneusta. Highly modified, no 
doubt, the adult animals are, but not degenerate. For these 
reasons the presumption of universal degeneracy on the part 
of all the lower Chordata will be dismissed, and an attempt 
made to systematize the facts as they are found. 
