142 The American Geologist. Septembor, i896 
the chain of forms already mentioned, to which many others 
may be added, there is only one rational explanation, namely, 
that the various links in them have arisen one from another, 
and are connected by blood-relationship. Moreover, the simi- 
larity of the faunas and floras which are nearest to one another 
in geological age, as well as the geographical distribution of 
extinct and still existing plants and animals, can only be sat- 
isfactorily explained on the assumption of descent. 
But, although an abundance of pabeontological facts can 
be cited in the most convincing manner in favor of the theory . 
of descent, on the other hand we must not forget that we 
know no point of origin for numerous independently arising 
creatures, find that the connection between the larger divisions 
of the animal and vegetable kingdoms is by no means so inti- 
mate as the theory especially postulates. The jubilation 
with which the discovery of Arc/ni^o/iferi/x was greeted at the 
time shows at best that links had previously been wanting 
between two classes which among vertebrates undoubtedly 
exhibited the closest relationships. Further, Archwopteri/x 
fills the gap between birds and reptiles only in a verj'- imper- 
fect manner, and affords no indication of the point at which 
the former have branched from the latter. It may, indeed, 
be maintained that we find ourselves to-day in greater uncer- 
tainty as to the origin of birds than we were twenty-five years 
ago, when Huxley's brilliant researches on the pelvis of the 
Dinosauria seemed to have found the bridge between the two 
classes. Links between the Amphibia and Reptilia are also 
still wanting. Perhaps they are to be found among the varied 
Theromorpha, but as yet palaeontology cannot determine the 
phylogenetic modification of the amphibian into the reptilian 
type. No zoologist will deny that the Mammalia hold an en- 
tirely isolated position, separated by a wide gulf from birds, 
reptiles, amphibians and fishes; while among all known mam- 
mals it is not some old fossil genus, but the duck-bill, still 
living in Tasmania, which most reminds one of the more lowly 
organized vertebrates. Certainly we still know too little about 
the skeleton of the Mesozoic mammals, and especially the 
Allotheria, for us to form a final opinion on this point. But 
the warmest adherents of the theory of descent must at all 
events admit that extinct links between the different classes 
