370 SCOTT. [Vol. V. 



down for determining the taxonomic value of any particular 

 structure, whether it is adaptive or inherited, positive or nega- 

 tive. In this respect every group must be considered by itself, 

 neglecting nothing. Fiirbringer has reached very similar results 

 in his great work on the birds : " Unter diesen Umstanden 

 muss die Beurtheilung dieser heterogenen Kennzeichen und die 

 Auslese unter ihnen filr jede Gruppe von Anfang an unter den 

 bereits angegebenen Cautelen getroffen werden und nirgends 

 darf man vergessen dass dieses oder jenes Merkmal, das sich 

 bei 10 Familien als ein constantes bewahrte, bei einer n ten eine 

 ganz uberraschende, seine taxonomische Anwendbarkeit sehr 

 beeintrachtigende Fliissigkeit darbieten kann " (No. 19, p. 1 134). 



4. Palaeontology is not well adapted to solve the fourth 

 problem, as to whether lost structures may be regained, because 

 we never can be sure that any case which might seem to require 

 such an interpretation is not really due to an imperfection of 

 the geological record. The apparent reacquisition of a digit, for 

 example, might be due to the fact that the ancestor of the poly- 

 dactyl form had not been preserved. There is, however, one 

 case which is of interest as indicating possibilities in this direc- 

 tion. One of the most characteristic features of the Oreodontidcz 

 is the fact that the first inferior premolar has assumed the form 

 and function of the canine, while the latter is functionally one 

 of the incisors. This condition is obviously a secondary one 

 and implies a change from the ordinary relations of these teeth. 

 On the other hand, in Pithecistes, one of the later and more 

 aberrant members of the family, the caniniform premolar has 

 disappeared and the canine has again taken on its normal func- 

 tions. This is, of course, not exactly a case in point, as the 

 canine was not lost and reacquired, but it would seem to indi- 

 cate that such reacquisition is not impossible. If Weissman's 

 interpretation of the relations of Siredon to Amblystoma be cor- 

 rect, then this is a clear case of a lost organ being reacquired ; 

 e.g. the gills, although these organs are normally present and 

 functional in the larval state of Amblystoma (No. 58). At all 

 events, such reacquisition, among the higher animals at least, 

 would certainly seem to be unusual, and should not be assumed 

 in any given case, except upon the clearest evidence. 



5. So far as the series of fossil mammals which we have been 

 considering are concerned, the developmental history appears to 



