EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 153 
can hardly be any more doubt but that we may look upon 
all the diverse modifications of lungs and swimming-bladder 
(the latter either double, ventral, or single and dorsal) as 
derivates of what were originally a pair of posterior gill- 
pouches, in which change of function was slowly inaugurated 
parallel to preparatory steps by which an adaptation to 
terrestrial life was rendered possible. 
Now the structures here alluded to are found in the Teleo- 
stomes, the Dipnoi, the Amphibia, Sauropsida, and Mammals, 
and never was any trace of them found in the Hlasmobranchs 
or the Cyclostomes; so that here we have a concomitant 
argument to the one derived from the trophoblast in further 
justifying the new line of demarcation. 
And I would call the attention of those who hesitate to 
introduce this new barrier between cartilaginous and osseous 
fishes to a set of other considerations which in my opinion 
have not been sufficiently looked into up to now. 
Tt is this, that while nobody objects to the Cetacea being 
looked upon as the descendants of terrestrial Mammalia, nor 
to the Sauropterygia and Ichthyopterygia as having sprung 
from Reptilia that were air-breathing land-animals, the 
question has not enough been looked in the face whether 
many of our Dipnoi, Ganoids, and Teleosts may not also 
perhaps have had terrestrial ancestors? I fully recognise 
that we are here entering a field of wild and hypothetical 
speculation, but on the other hand insist on the necessity 
of testing this heuristic assumption. If we admit that air- 
breathing, hairy, and milk-producing quadrupeds originally 
lived on the dry land and have been able secondarily to 
adapt themselves in the most marvellous way to a life abso- 
Iutely bound in all its functions to the high seas as that 
of the whales, how could we then wonder that in the 
palzeozoic epoch, when for the first time life on the dry land 
became possible and weird amphibious protetrapods left the 
water and managed to adapt themselves to this atmospheric 
environment, on many an occasion side branches of these 
earliest land-animals turned back ‘to purely aquatic life 
