30 PROFESSOR A. M. MARSHALL. 



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 mio-ht 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 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 degeneration to have occurred 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 invoke degeneration 

 rather freely as a talisman to extricate us from morphological diffi- 

 culties ; and an inclination to accept such suggestions, at any rate 

 provisionally, without requiring satisfactory 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. 



