JtEFliODUCTIVE CYCLE IX DASYURUS VlVERlilNUS. 171 
We have had no experience of Didelphys^ but cannot regard 
Selenka’s statement as indubitable in the absence of any 
record of the condition of the ovaries, wliilst as regards 
Macropus spp., we can only say that our records indicate that 
in New South Wales at all events they certainly breed during 
the summer months (December to February). Irrespective, 
however, of the number of breeding seasons pei- year, we 
have no evidence as to whether the macropods are moncestrous 
or not. 
As regards the Monotremes, our own experience confirms 
the statement of Caldwell (5) and Semon (26)^ to the effect 
that the breeding season recurs but once annually. In view 
of the facts that at most two eggs in the single functional 
ovary reach maturity at the same time, and that the breeding 
season is of such short duration that it would appear to be 
impossible for a second set of eggs to become full-grown 
within its limits, we consider it justifiable to assume that 
Monotremes are, like Dasyurus, moncestrous. 
In view of these considerations and of others relative to 
the breeding habits of reptiles which we do not think it 
necessary to bring forward here, and in view, moreover, of 
the lowly position occupied by the monotremes and marsupials 
in the mammalian series, we find it difficult to avoid the 
conclusion that the moncestrous condition is the primitive one, 
and that the polycestrous condition has been secondarily 
derived from it. This latter condition where it occurs 
amongst the Eutheria is obviously advantageous, since it 
permits of the production of a larger number of young, or at 
least provides greater opportunity for successful impregnation. 
List of Keferences. 
1. Ancel, P., et Bouin, P. — “ Sur les Homologies et la Signification des 
Glandes a Secretion interne de I’ovaire,” ‘ Compt. rend. Soc. Biol.,’ 
t. Ixvii, 1909. 
^ Semon states (p. 8) that “die Monotremen haben in jedem Jalire 
nnr eine Brunst.” 
