236 ANIMAL PEDIGREES 



at present a tendency to invoke degeneration rather 

 freely as a talisman to extricate us from mor- 

 phological difficulties ; and an inclination to accept 

 such suggestions, at any rate provisionally, without 

 requiring satisfactory evidence in their support. 



Degeneration of which there is direct embryo- 

 logical 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 sign 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 development, con- 

 cerning which, thanks to the patient investigations 

 of Kowalevsky and Hatschek, our knowledge is 

 precise, there is no animal known to us in which 

 the sequence of events is simpler or more straight- 

 forward. Its various organs and systems are 

 formed in what is recognised as a primitive manner ; 

 and the development of each is a steady upward 

 progress towards the adult condition. Food yolk, 

 the great cause of distortion in development, is 

 almost absent, and there is not the slightest indi- 



