PEDIGREE OF MAMMALS. 



271 



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 en- 

 title 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 as- 

 sume 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 impossi- 

 bihty. 



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 

 mammals, the vascular processes and villi of the wall 



