PLATYPUS AND ECHIDNA 
tively mammalian but appertain to the lower lying 
amphibians and reptiles, are important indications of 
the grade of the mammal concerned. Now perhaps no 
two mammals are more diverse in outward appearance 
and even in many anatomical details than the spiny 
ant-eater of Australia, and the duck-billed platypus 
of the same continent. 
The one is spiny, toothless, long-snouted, long- 
tongued ; the other furry, ‘“ duck-billed,”’ web-footed ; 
their skeletons and internal organs differ widely, and 
yet in both of these creatures the young are hatched 
from large eggs with much yolk, and there is no con- 
nexion between the young and the mother, the eggs 
leaving the body as eggs. Furthermore, there is a 
peculiar vein, or traces of it, which is quite like a vein 
found in the lower vertebrata, but not represented in 
the higher mammals, and the structure of the heart shows 
an analogous state of affairs. All these points link the 
Monotremata, as the order comprising these the only 
two existent types of the order is called, together ; they 
have been arrested, as it were, before they have com- 
pletely thrown off the characters of the amphibio- 
reptile from which the mammalia have in course of time 
beenevolved. Wemust therefore clearly set apart those 
two forms into a group by themselves. The remaining 
mammals, the vast majority of course, can hardly be 
sorted into an ascending series, getting less and less 
reptilian. Paleontology, moreover, while explicit as to 
the relationships of certain groups, is at present cloudy 
as to the derivation of the higher mammals, or Eutheria 
as they have been termed, from the Monotremata, the 
only surviving members of the early mammals, or 
Prototheria. All the Eutheria contrast with the 
Prototheria in having minute eggs which are not “ laid,” 
but develop inside the body of the parent, and come 
to be attached to the tissues of the mother, deriving 
Tide 17 c 
