234 ANIMAL PEDIGREES 



recognised by Darwin as a possible, and under 

 certain conditions a necessary consequence of his 

 theory, has been since advocated strongly by Dohrn, 

 and later by Lankester, in an evening discourse 

 delivered before the British Association at the 

 Sheffield meeting, in 1879. Both Dohrn and 

 Lankester have suggested that degeneration may 

 occur much more widely than is commonly sup- 

 posed. 



In animals which are parasitic when adult, but 

 free swimming in their early stages, as in the case 

 of Sacculina, 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 noto- 

 chord, 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 organisation, 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 accept- 

 ing a group as degenerate. We are reminded of 

 the tendency to abbreviation or to complete omis- 

 sion of ancestral stages of which we have quoted 

 examples above ; and it is suggested that if such 



