88 DR. B. A. BENSLEY OX THE EVOLUTIOX 



the adaptive cliaractevs as presented by existing animals. Objection may be justly taken 

 on general principles to basing phylogenetic conceptions on the characters of living forms, 

 not only on account of the necessity it involves of selecting ancestral and derived 

 characters from contemporaneous types, and thereby incurring a risk of mistaking the 

 direction in which the evolution may be proceeding, but also on account of the necessity 

 of recognizing hypothetical collective forms. Many instances might be cited in which 

 the identification of such forms lias not got beyond their original creation. While 

 preferring to claim too little than to assert too much in favour of the stability of 

 the present arrangement, the writer believes that such difficulties as those referred to 

 are. in the case of the Australian radiation, more than compensated for by other 

 advantages. It may be observed, in the first place, that in utilizing the characters 

 of the dentition and foot-structui-e we are dealing with organs whose adaptive modi- 

 fications are, to a much greater extent than those of others relating to the internal 

 organization, of a definite and irreversible stamp. The Australian Marsupials represent, 

 furthermore, a peculiarly homogeneous group. During their development they have 

 been protected from competition with other animals. They have apj)arently been free 

 from such disturbing conditions as result from the development of a large carnivorous 

 element. Their range of lateral radiation has been restricted by their geographical 

 limitations ; so that their progressive development has not been greatly disturbed by 

 divergence. Finally, their evolution has not proceeded to the stage of over-population 

 and the obliteration of less specialized intermediate types. Under these conditions we 

 may expect to find — as, except in a few cases, we actually do — that the adaptive modi- 

 fi(;ations follow one another witli a precision which leaves no doubt as to the manner in 

 wliich the evolution began and the direction in which it is proceeding. 



It will be borne in mind, however, that the present arrangement is morphogenetic 

 rather than truly j)hylogenetic, because, while doubtless supported in different ways by 

 tlie modifications of other organs *, it is based solely on the dentition and foot-structure, 

 Avhereas true phylogeny must account for the modification of all parts of the organization. 

 Hence, while genera or species indicated as ancestral or derived may be actually so in a 

 phylogenetic sense, they are only advanced as such so far as tlie morphological differen- 

 tiation of their dentition and foot-structure is concerned. 



The Adaptive Modifications of the Dentiliox in the Avstraliax 



Marsupials. 



The following details of nomenclature may be noted : — 



With reference to the cheek-teeth, it is a familiar fact that in the Marsupials the 

 numbers of premolariform and molariform teeth are the reverse of those in the Placentals, 

 being respectively 3 : 4 in the former and 4 : 3 in the latter. The question of their serial 

 homologies is one which has given rise to considerable controversy ; and several plans 

 for the nomenclature of marsupial teeth have been proposed. A consideration of the 



* a paper on the general correlation of changes in the mammary apparatus is in course of preparation. 



