158 A. A. W. HUBRECHT. 
already placental Monodelphia, so that the Mammalia s. str. 
are no more broken up into three stems but in reality contain 
only one, the course of which through the corridors of time 
will have to be established by the palzontologists, who will 
undoubtedly finally be able to trace it far into the carboni- 
ferous, nay, perhaps, even into earlier geological epochs, 
simultaneously with the first evolution of air-breathing verte- 
brates of Protetrapodian structure. 
Then, again, if we take these Monodelphia, their respective 
subdivision into natural orders will awaken all the more 
interest as they bring us closer to the phylogenetic develop- 
ment of man himself, one of the problems about which the 
human mind will never be wholly at rest ; and here compara- 
tive anatomy, embryology and palxontology ought to co- 
operate more intensely than it has hitherto generally done. 
Only of late—thanks, in the first place, to efforts of American 
paleontologists—this is brought home to us and is begining 
to be realised. 
Here, too, however, a very broad and very modern spirit 
ought to prevail. And though recognising that only of the 
recent Mammalia the embryology can be traced, and that 
there is not the least chance of ever obtaining positive facts 
concerning the embryology of fossil groups, still, it ought to 
be fully realised that when once the ontogeny of all the 
existing genera of mammals is known—and this is a goal that 
ought to be taken in view without delay—we will have in 
those facts indications of great delicacy for determining 
degrees of consanguinity. The details of ontogeny and 
fully known, these ancient fossil types will present every conceivable grada- 
tion between these two great divisions of the Vertebrata.” 
Now, this is the very point which, on repeated occasions (’95, ’02), I have 
advocated, viz. that the recent Ornithodelphia are one of the many offshoots 
into which the Protetrapodan ancestors have subdivided themselves, when 
once they had commenced to adapt themselves to life on dry land and to aerial 
respiration. The stems that remained viviparous are yet represented by the 
living Mammalia, those that have become oviparous diverged into the Orni- 
thodelphia, and—further off yet—into numerous Reptilia, and have never 
given rise to viviparous descendants. 
