THE EMBRYOLOGY OF THE OPOSSUM 207 



Bensley ('Ola, '01 b, '03) points out that as far as the diphyo- 

 dont condition of the Metatheria is concerned, that is exactly what 

 Huxley hypothesized in his original paper (1880, p. 655) ; and it 

 seems to be well substantiated by the finding of a suppressed milk 

 dention among modern marsupials. But as to the placental condition, 

 only one marsupial is known to have an allantoplacenta comparable to 

 that of Eutheria, and that marsupial, Perameles, is one of the more 

 specialized forms, not a very primitive one. Accordingly, he thinks 

 it more likely that this is a case of evolutionary convergence. 



Flynn ('22) argues that the extraordinary similarity of the allanto- 

 placenta in Perameles to that in the Monodelphia makes convergence 

 seem an inadequate explanation, and that all of the aplacental mar- 

 supials (i.e., most of the marsupials) must be degenerate in this re- 

 spect, not primitive. This seems unsound for two reasons. There 

 are too many other instances of striking convergence between the 

 marsupials and the placental mammals ; and there are too many other 

 anatomical indications that the most primitive marsupials are 

 aplacental. In a similar way one might argue that the extraordinary 

 similarity between the incisors of the Diprotodontia and those of 

 the Rodentia proves that the marsupials are derived from rodent 

 ancestors, and that all of the Polyprotodontia are degenerate, not 

 primitive. 



Striking examples of homoplasy or convergence are not to be 

 wondered at among members of a single Phylum and Class. If the 

 eye of a mollusc is found to be identical in almost every detail with 

 the eye of a vertebrate, one may justifiably express consternation ; but 

 among animals universally believed to have had common ancestors 

 at a not very remote geological period, this kind of convergence is 

 not only natural, but common. The detailed parallelism between the 

 adaptive radiation of the Australian marsupials and that of the mono- 

 delphia, which has been known ever since the days of Cuvier, fur- 

 nishes numerous instances in point. Witness the extraordinary simi- 

 larity between the marsupial mole and the eutherian mole, the 

 marsupial rodent and the eutherian rodent, the marsupial cats and 

 the eutherian cats, the flying phalangers and the flying squirrels, etc. 



In regard to the evidence that the most primitive marsupials are 

 aplacental, we may ask what features we may regard as indications of 

 primitiveness. If we take the same criteria which have been used in 

 working out the evolutionary tree of the Eutheria, we can establish a 

 very similar tree for the marsupials. 



Dollo (1899) showed very convincingly that the feet of all modern 

 marsupials can be derived from an ancestral form very remarkably 

 like that of the modern opossums (Didelphyidae), but that even this 

 primitive foot pattern included one specialized feature the hallux of 



