26 NATURAL SCIENCE. January. 



conclusion that the date of the appearance of this fauna is enormously- 

 removed from the beginning of life, because it contains highly- 

 specialised forms, must be taken with considerable abatement. 



Turning now to the question whether we can legitimately 

 assume that degeneracy and differentiation have alternated in the 

 history of a species — in a word, whether degenerate ancestors have 

 given rise to active forms with highly developed sense-organs — let us 

 glance at those cases where all will admit that a certain amount of 

 recovery has occurred. One of the best-known cases is that of 

 Pecten. There is strong reason for believing that all lamellibranchs 

 are descended from more active forms provided with cephalic eyes 

 and tentacles, and separate cerebral and pleural ganglia. Owing to 

 their sluggish, burrowing mode of life, and gross non-selective feeding, 

 these features have been lost. The oysters, to which Pecten is not 

 distantly allied, have carried this degeneracy to its extreme point — 

 the foot, one adductor muscle, and the pedal ganglia being likewise 

 absent. Pecten has recovered the free-swimming life, and has developed 

 a new set of sense-organs ; but none of the lost structures have been 

 recovered. I know of no other case where it is at all probable that 

 recovery has taken place, and there is no ground for assuming that 

 burrowing forms could ever give rise to active descendants. This 

 brings us to the question, how primitive or undifferentiate can be 

 distinguished from degenerate features. How are we to say, for 

 instance, what features in Ainphioxits we may regard as ancestral, and 

 what as secondary ; or in Pevipatus or Chiton ? I do not assert that 

 any absolute criteria can be put forward ; each case must be judged 

 on its merits : but certain suggestions can be made. The criterion of 

 primitive characters is their synthetic character — that is, they serve 

 to link together different groups. Why are the gill and foot of 

 Nncnla said to be of a primitive character ? Because they agree, in 

 contradistinction to those of the lamellibranchs, with the gastropod 

 gill and foot. An organ may be said also to be primitive if it 

 combines functions distributed in other animals amongst different 

 sets of organs. Thus, the coelom of brachiopods combines the 

 functions of excretory organ, generative organ, and body-cavity : 

 so we regard it as being in a very primitive condition. Under- 

 lying such reasoning there is a principle I believe to be sound ; 

 it is that new organs are never developed from functionless 

 rudiments — as Darwin at one time thought himself forced to believe 

 — but arise by modification of pre-existing organs. Where the 

 function of an organ is changed, the newer function must have existed 

 with, but subsidiary to, the old one. Eyes and ears would thus be 

 local spots of peculiar sensitiveness on a skin which responded to both 

 light and sound. Degenerate organs are no doubt simplified, but 

 not only are they not synthetic in the sense mentioned above, that 

 they do not recall the conditions of affairs in other groups, but they 

 are not correlated with the state of development of other organs. A 



