PEDIGREE OF MAMMALS. 271 
and loops, between which grow similar offshoots and 
appendages of the blood-vessels. of the ovary, so that 
through. the walls of the contiguous blood-vessels an 
abundant exchange of fluids takes place between the 
two, and therewith a prolonged nutrition and a further 
and more complete development of the foetus. The 
higher character of the placental mammals, usually 
plainly evinced by their anatomical relations, is thus 
based on the existence of the placental mass, All inter- 
mediate grades are, however, wanting which would 
entitle us to infer with certainty the direct transition 
from implacental to placental mammals. The Edentata, 
(Bruta), manifestly the lowest of placental mammals, 
are so devoid of any nearer morphological relations 
with the Marsupials, that we must needs be content to 
assume generally, on these indications, supported by 
geographical distribution and geology, that they repre- 
sent a very ancient branch of the placental mammals. 
As we saw in the tenth chapter, they are scattered 
remnants which can only by compulsion be united 
into a single order. Sloths, armadilloes, ant-eaters, 
differ from one another at least as much as rodents, 
insectivora, and bats. The doctrine of Descent is not 
discredited because it is unable to account for these 
fragments of bygone animal life, but in the absence 
of data it is for the time in presence of an impos- 
sibility. 
To ascertain the relationships of other orders, the 
modern systematizers, and also the supporters of the 
system of Descent, have thought fit to lay great stress 
on the. presence or absence of the so-called decidua. 
This requires a short explanation. In many orders of 
