3 8 THE INTERNATIONAL MONTHLY. 
the monotremes, which are the remnant of an extremely ancient 
type. It follows from this that Huxley's division of the mam- 
mals into three sub-classes must be discarded in favor of Gill's 
arrangement into two sub-classes, the Prototheria (monotremes) 
and Eutheria (marsupials and placentals) ; an arrangement which 
conforms better to the facts. 
Throughout the whole of the Mesozoic era the mammals 
continued to be very rare, very small, and very inconspicuous, at 
least in the regions which are geologically known. They were 
completely overshadowed by the reptiles, which, in such aston- 
ishing numbers and variety dominated the land and the sea 
and even the air. It is, of course, possible that in some region 
of the world, in the later Mesozoic, the mammals may have 
been much further advanced in size, in numbers, and in differen- 
tiation than the fossils of Europe and North America would lead 
us to expect. Indeed, there is some reason to believe that such 
was actually the case, and that the wonderful expansion and 
development of mammalian life, which characterized the older 
Tertiary times of the northern hemisphere, was due rather to 
immigration than to evolution within that hemisphere. 
Even in the northern hemisphere, however, the mammals made 
considerable progress during the Mesozoic era. Most unfor- 
tunately, all of the fossils yet obtained are so fragmentary, con- 
sisting almost entirely of isolated jaws and scattered teeth, that 
they teach us comparatively little concerning themselves or their 
relationships. Almost all of minute size, their extreme rarity is 
not to be wondered at. Hence conjecture plavs a large part in 
the deductions which are drawn from the study of these most 
interesting but unsatisfactory fossils. Judging from the best 
evidence now available, it would appear that within the limits of 
the Mesozoic era both of the mammalian sub-classes, the Pro- 
totheria and the Eutheria, had become established, and further, 
that the two primary divisions of the latter, the marsupials and 
placentals, had already been separated. It is even probable that 
the beginnings of several of the modern mammalian orders date 
back to the latter part of the Mesozoic. 
