1898] AXXCLA '/'A' , t X( 'E8T1! Y OF THE VEBTEBRATA 2!> 



their normal method, and by becoming highly specialised may have 

 carried those who were otherwise on the upward path of vigour and 

 rapacity back into a quieter life. We may quite agree with Brooks 

 that the free-swimming tunicates show no sign of degeneration, that 

 is. judged by themselves, and we may admit their perfect adaptation 

 to their method of life ; but, none the less, according to our argu- 

 ment, the line of their development leads to the sedentary tunicates. 

 Further, if there is any truth whatever in our sketch of the rise of 

 the notochord, we must believe that the ancestors of Appendicular ia 

 once chased and seized prey with their buccal teeth, and swallowed 

 more or less solid food, leading to periodical distension of the 

 alimentary canal. All this has been lost; the seizing apparatus has 

 vanished, and the alimentary canal requires to put forth no further 

 serious digestive efforts, as the food-particles which dribble to it are 

 easily disposed of. 



We should then perhaps have to regard Amphiorus and Appendi- 

 cularia as two distinct and separate offshoots from the advancing 

 army of the new race/ in both cases enticed, as it were, on one side 

 to abandon the rapacious method of feeding for the easier and more 

 passive method above described. 



I have already suggested that no fossil transition forms between 

 hirudineans and cyclostomes could be expected, and as a matter of 

 fact, on turning to palaeontology, we find that when the palaeozoic 

 vertebrates first appear they appear suddenly, as primitive fish without 

 jaws or paired limbs. Only one form, Palaeospondylus of Traquair, 

 from the Old Bed Sandstone, is claimed to have been a cyclostome 

 or lamprey. That only one such form is found, while the rest are 

 shark-like in shape with heavy armour, does not seriously affect our 

 argument. For we have no difficulty in assuming that the lamprey 

 with its unarmoured skin is more primitive than these shark-like 

 forms with their heavy defensive armour. Further, I should con- 

 clude that these armoured sharks implied the presence of un- 

 armoured forms, which for that reason have left no remains. I 

 should regard the armoured forms as having arisen as offshoots from 

 the active advancing race — offshoots which have retired to a more 

 peaceful life behind their secondarily acquired defences. I feel 

 obliged to assume that these armoured sharks were so far degen- 

 erate forms and were not the ancestors of the later unarmoured 

 true sharks, because I think that heavily-armoured creatures have 

 practically ceased to advance. According to the principles here 

 adopted, that the acquirements of new functions lead to structural 

 adaptations, we see that passivity, or even activity if limited to 

 narrow grooves, must be fatal to evolutionary progress. The 

 Gigantostraca which have died out, with the exception of the 

 stationary Limulus, are (pace Gaskell) examples of this. Further, 



