88 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 Echiurus, &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. 



