334 THE RECAPITULATION THEORY 



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 organisation, at a distinctly 

 lower morphological level, is less highly differen- 

 tiated 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 

 to abbreviation or to complete omis- 



sion 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 de- 

 generation 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 acqui- 

 sition of food yolk, then it is urged that the 

 degenerate condition of the group might easily 

 escape detection. The supposition is by no means 

 extravagant ; food yolk varies greatly in amount 

 in allied animals, and cases like Hylodes, or 

 amongst Ascidians Pyrosoma, show how readily a 



