164. DR. B. A. BENSLET ON THE EVOLUTION 



follows : — The first by the species of Sminthopsis and by Antechinomys laniger; a second 

 by Chmtocercus cristicauda and Dasyuroides Bi/rnel; a third by Dasyiirus Geoffroyi 

 and I), viverrinus ; and a fourth by Sarcophilus ursiims. The species described by 

 Spencer (1896) as Fhascogale macdormellensis may represent another terrestrial line, but 

 the animal is doubtfully separable from SmintJwpsis. The genera Myrmecohius and 

 Thylacinus represent two more terrestrial lines whose exact connections are doubtful. 



The above reference to the main line of the evolution as arboreal or semi-arboreal 

 calls for some explanation. It will be seen from a general survey of the family that 

 even in the most primitive species the hallux, although of an opposable type, is much 

 smaller than in typical arboreal forms, such as tliose of the Phalangeridae and 

 Didelphyidse, and the digits present a parallel ratber than a primitive radiate arrange- 

 ment. Thomas (i B88) remarks that the species of Phascogale are strictly arboreal, while 

 Wiuge (1893) states tliat the foot of Phascogale is more modified for terrestrial service 

 than in the Didelphyidoe. 



The fact tliat the forms here referred to as constituting sviccessive lines of terrestrial 

 evolution present definite terrestrial characters, shows that modifications presented by 

 Fhascogale must be explained on some other assumption than tbat they represent 

 terrestrial adaptations. 



It is a well-known fact that the Didelphyidse, which approximate very closely to 

 the present family, present typical prehensile developments both in the pes and in 

 the tail. In the Dasyuridte these prehensile developments are absent, if we except the 

 presence in some of striated plantar pads and the opposability of the reduced hallux. 

 The conditions in the Dasyuridoe thus appear at first sight to indicate that the family 

 has been derived originally from forms which had already embarked on a terrestrial 

 evolution. This proposition may, however, be dismissed with the simple statement, since 

 in another section of the Australian fauna, namely the Phalangeridae, we meet with 

 hallucal characters of the same primitive arboreal stamp as those of the Didelphyidae. 

 The alternative explanation is that the Dasyuridje have abandoned modifications of 

 prehensilism for others better suited for either arboreal or terrestrial progression in 

 connection with their insectivorous liabits. Comparison with the Placentals shows that 

 arboreal habit does not necessarily demand the adoption of prehensilism, although it is 

 indicated to a marked degree in some forms (Primates), as it is in the Marsupials. The 

 modifications of the pes in the primitive Dasyuridse indicate a cursorial development 

 involving elongation of the pes and parallel arrangement of the digits as well as 

 recession of the hallux. The ancestral forms of the family appear to have abandoned 

 their prehensile modifications for semi-cursorial ones of undoubted advantage in 

 insectivorous life, the pes having thus been made serviceable for rapid progression 

 either in the trees or on the ground. At the same time they have lost nothing in the 

 abandonment of modifications of prehensilism, these being unnecessary in animals 

 which are capable of balancing their bodies in arboreal progression by means of 

 rapid locomotion. Those of their successors which have become definitely terrestrial 

 are not necessarily more completely cursorial, although their special characters are 

 largely the result of a substitution of digitigrade for plantigrade progression. 



