842 REPORT—1890. 
commonly called degenerate. The principle of degeneration, recognised by 
Darwin as a possible, and, under certain conditions, a necessary consequence of his 
theory of natural selection, has been since advocated strongly by Dohrn, and later 
by Lankester in an Evening Discourse delivered before the Association at the 
Sheffield Meeting in 1879. Both Dohrn and Lankester suggested that degeneration 
occurred much more widely than was generally recognised. 
Tn animals which are parasitic when adult, but free swimming in their early 
stages, as in the case of the Rhizocephala whose life history was so admirably 
worked out by Fritz Miiller, degeneration is clear enough: so also is it in the case 
of the solitary Ascidians, in which the larva is a free swimming animal with a 
notochord, an elongated tubular nervous system, and sense organs, while the adult 
is fixed, devoid of the swimming tail, with no notochord, and with a greatly 
reduced nervous system and aborted sense organs. 
In such cases the animal, when adult, is, as regards the totality of its organ- 
isation, at a distinctly lower morphological level, is less highly differentiated than it 
is when young, and during individual development there is actual retrograde 
development of important systems and organs. 
About such cases there is no doubt: but we are asked to extend the idea of 
degeneration much more widely. It is urged that we ought not to demand direct 
embryological evidence before accepting a eroup as degenerate. We are reminded 
of the tendency to abbreviation or to complete omission of ancestral stages of which 
we have quoted examples above; and it is suggested that if such larval stages 
were omitted in all the members of a group we should have no direct evidence of 
degeneration in a group that might really be in an extremely degenerate condition. 
Supposing, for instance, the free larval stages of the solitary Ascidians were 
suppressed, say through the acquisition of food yolk, then it is urged that the 
degenerate condition of the group might easily escape detection. The supposition 
is hy no means extravagant ; food yolk varies greatly in amount in allied animals, 
and cases like Hylodes, or amongst Ascidians Pyrosoma, show how readily a 
mere increase in the amount of food yolk in the egg may lead to the omission of 
important ancestral stages. 
The question then arises whether it is not possible, or even probable, that 
animals which now show no indication of degeneration in their development are in 
reality highly degenerate, and whether it is not legitimate to suppose such degenera- 
tion to have uccurred in the case of animals whose affinities are obscure or difficult 
to determine. 
It is more especially with regard to the lower vertebrates that this argument 
has been employed; and at the present day, zoologists of authority, relying on 
it, do not hesitate to speak of such forms as Amphioxus and the Cyclostomes 
as degenerate animals, as wolves in sheep’s clothing, animals whose simplicity 
is acquired and deceptive rather than real and ancestral. 
I cannot but think that cases such as these should be regarded with some 
jealousy: there is at present a tendency to inyoke degeneration rather freely 
as a talisman to extricate us from morphological difficulties; and an inclination 
to accept such suggestions, at any rate provisionally, without requiring satis- 
factory evidence in their support. 
Degeneration of which there is direct embryological evidence stands on a very 
different footing from suspected degeneration, for which no direct evidence is 
forthcoming ; and in the latter case the burden of proof undoubtedly rests with 
those who assume its existence. 
The alleged instances among the lower vertebrates must be regarded 
particularly closely, because in their case the suggestion of degeneration is 
admittedly put forward as a means of escape from difficulties arising through 
theoretical views concerning the relation between vertebrates and invertebrates. 
Amphioxus itself, so far as I can see, shows in its development no sien of 
degeneration, except possibly with regard to the anterior gut diverticula, whose 
ultimate fate is not altogether clear. With regard to the earlier stages of develop- 
ment, concerning which, thanks to the patient investigations of Kowalevsky and 
Hatschek, our knowledge is precise, there is no animal known to us in which the. 
