84 



DK. B. A. BEXSLET ON THE EVOLUTION 



into a line of descent, the lowest being associated with the E.eptilia and indirectly with 

 the Amphibia, or directly with the latter group. It has been found, however, that the 

 Monotremata do not occupy a wholly prototypal position with reference to the 

 Mar.suj)ialia, and that the latter are by no means wholly prototypal to the Placentalia ; 

 and the necessity has thus arisen for the recognition of hypothetical or ideal groups 

 through which the evolutionary sequence might be assumed to have been established, 

 and from which the existing groups might be supjjosed to have arisen by independent 

 specialization. The clearest exposition of tliis principle is to be found in Huxley's essay 

 of 1880, in which three type groups are characterized and designated as Prototheria, 

 Metatheria, and Eutheria, these being assumed to have arisen in the order named, and 

 to have given rise successively to the Monotremata, Marsujoialia, and Placentalia. 



Monotremata. 



Marsupialia. 



Placentalia. 



Eiitheria. 



Metatheria. 



Prototheria. 



A phylogenetic plan, sho\ving the primary relationships of the Mammalia. 



Naturally, the Eutherin, being a terminal group, are practically equivalent to Placentalia. 

 Huxley remarks : — " It is a fact, curiously in accordance with what might be expected 

 on evolutionary principles, that while the existing members of the Prototheria and the 

 Metatheria are all extremely modified, there are certain forms of living Eutheria wliich 

 depart but little from the general type. . . . There is no known Monotreme which is not 

 vastly more different from the Prototherian type, and no Marsupial which has not far 

 more widely departed from the Metatherian type, than Gymnura or, indeed, Erinaceus 

 have from the Eutherian type " *. 



* Careful analysis shows that it is not absolutely necessary to recognize more than two primary mammalian 

 groups, one including the stock-forms of the Monotremata, the other those of the Marsupialia and Placentalia. The 

 (luestion arises, How are these groups to be designated ? and this leads to the further question, In fixing the 

 designations, are we to rely on strict priority or on common usage? As early as 1872 Gill divided the Mammalia 

 into two primary groups of Prototheria and Eutheria; and this division has been used by Cope (1889) and recently 

 by Osborn (1899). Uills Eutheria were not assigned a placental or aplaccntal character. Common usage has it that 

 the Eutheria are placental, as specified by Huxley. It accordingly follows that if we revert to GilFs classification, 

 we must characterize the Eutheria as fundamentally equivalent to Huxley's Metatheria and as aplacental like the 

 majority of the Marsupialia, unless it is proved that the latter animals are of placental origin, as supposed by Wilson 

 and Hill, and Dollo. Even in the latter event, it will be apparent that allowance must be made for a Metatherian 

 stage in development, altliough not for a definite Metatherian group. 



