388 A. A. W. HUBRECHT. 
Deciduata, and, further, that the Lemuroids, notwithstanding 
their intermediate position (on other structural characters) 
between the deciduate Insectivora and Primates, are also 
non-deciduate, is of course not calculated to strengthen our 
conviction that the subdivision proposed by Huxley is on the 
whole a natural one. Turner, Ercolani, and Balfour have 
already laid stress on the fact that no sharp line can be 
drawn between the two types. 
The details of the histological structure of the trophosphere 
and placenta of a representative of the Insectivora above given 
tends to show that, moreover, our knowledge of the placenta- 
tion of the Deciduata is as yet not extensive enough to enable 
us to generalise with respect to it. More detailed researches 
will have to be entered upon, and I presume that in many 
instances a break-down of the classificatory significance of 
certain placentary characters may ensue, whereas, on the other 
hand, most important points concerning the special adaptative 
and developmental peculiarities of the placenta—that youngest 
organ in the mammalian economy—may yet be brought to light. 
The fact in itself of the great youth of the placenta as 
compared to the other chief components in the organisation of a 
mammal renders it extremely probable that amongst the more 
primitive orders of Monodelphia adaptative differences of the 
placenta may occur in closely allied forms. Evidence may thus 
come forward that natural selection has not yet been long enough 
at work to reveal its sifting influence in a pitiless elimination 
of certain placental adaptations which, in the long run, must 
prove less favorable than others. 
This probability verges upon certainty if we compare in 
detail what three representatives of the Insectivora—Erinaceus, 
Talpa', and Sorex—teach us in this respect. The two latter 
* On p. 346 it was stated that curiously divergent phenomena are noticed 
concerning the afterbirth of the mole as compared with that of the hedgehog. 
1 was much impressed to find a short passage in Owen’s ‘ Comp. Anatomy’ 
(vol. iii, p. 730) which runs as follows: “In the mole the placenta is a circular 
CISC 22a the linear track of the uterine surfaces to which the placenta is 
attached shows a fine areolar structure, penetrated by the foetal placentary 
