1898) ANNULATE ANCESTRY OF THE VERTEBRATA 29 
their normal method, and by becoming highly specialised may have 
earried 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 areu- 
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 Appendicularia 
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 Amphioxus 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, Palacospondylus of Traquair, 
from the Old Red 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, 
