364 A. A. W. HUBRECHT. 
abandoned. Here I will insist upon the intrinsic improbability 
that the successive steps of differentiation, along which the 
human placenta has gradually evolved out of one of the lower 
forms, should have been retained in orders so divergent and so 
specialised as are the Ungulates, the Carnivores and the 
Primates. We are all convinced that no actual line ever con- 
nected these three in the order indicated. On the contrary, 
that most important contribution to mammalian phylogeny, 
which we owe to Huxley (‘ Proc. Zool. Soc.,’ 1880, p. 649), 
specially insists on the fact that the existing orders all tend 
towards lower ancestors, from which the existing Insectivora 
appear to have diverged less than other orders; this order thus 
having to be regarded as that of the more primitive monodel- 
phian mammals. These conclusions are endorsed on account 
of personal investigation, along other lines, by W. K. Parker and 
many others, and also by palzontology, which finds insectivor- 
ous remains at the base of the monodelphian genealogical tree. 
If now, amongst the Insectivora, we find the phenomena of 
placentation far more complicated than in the Ungulates, and 
leading up more directly to what we find in the Primates and 
in Man, we may feel sure that we ought not to leave this track 
in determining placental phylogeny, and that the primitive 
stages that have served Turner as a starting-point for his 
speculations, should no longer be considered as such and 
a wide margin left for the possibility of coenogenetic sim- 
plification. We are thus compelled to a revision of the 
data that are connected with the phylogeny of the placenta. 
Turner’s diagrams were tempting because of their simplicity. 
The evolution of the arborescent villi of the human placenta, 
suspended in the maternal blood-lacune, out of the simple 
chorionic wart of the pig, appeared to be well founded enough, 
and a comparison of the epithelial layers was even confidently 
brought forward. The well-known prolongation of maternal 
epithelium, by which the arborescent villus of the human 
placenta was said to be invested, formed an important item in 
this reasoning. Four propositions, which follow from what I 
have demonstrated in the hedgehog, and which are substantiated 
