1885.] Progressive and Retrogressive. 349 
First, as to the feet, and (A) the digits. The Condylarthra 
have five digits on both feet, and they are plantigrade. This 
character is retained in their descendants of the lines of Anthro- 
poidea, Quadrumana and Hyracoidea, also in the Bunotheria, 
Edentata and most of the Rodentia. In the Amblypoda and 
Proboscidia the palm and heel area little raised. In the Carnivora 
and Diplarthra the heel is raised, often very high, above the 
ground, and the number of toes is diminished, as is well known, to 
two in the Artiodactyla and one in the Perissodactyla. (B) The 
tarsus and carpus. In the Condylarthra the bones of the two 
series in the carpus and tarsus are opposite each other, so as to 
form continuous and separate longitudinal series of bones. This 
continues to be the case in thé Hyracoidea and many of the 
' Quadrumana, but in the anthropoid apes and man the second row 
is displaced inwards so as to alternate with a first row, thus inter-. 
rupting the series in the longitudinal direction, and forming a 
stronger structure than that of the Condylartha, In the Buno- 
therian rodent and edentate series, the tarsus continues to be 
without alternation, as in the Condylarthra, and is generally 
identical in the Carnivora. In the hoofed series proper it under- 
goes change. In the Proboscidia the carpus continues linear, 
while the tarsus alternates. In the Amblypoda the tarsus alter- 
nates in another fashion, and the carpal bones are on the inner 
side linear, and on the outer side alternating. The complete 
interlocking by universal alternation of the two carpal series is 
only found in the Diplarthra. (C) As to the ankle-joint. In most 
of the Condylarthra it is a flat joint or not tongued or grooved. 
In most of the Carnivora, in a few Rodentia, and in all Diplarthra, 
it is deeply tongued and grooved, forming a more perfect and 
stronger joint than in the other orders, where the surfaces of the 
tibia and astragalus are flat. (D) In the highest forms of the 
Rodentia and Diplartha the fibula and ulna become more or less 
coossified with the tibia and radius, and their middle portions be- 
come alternated or disappear. 
Secondly, as regards the vertebra. The mutual articulations 
(zygapophyses) in the Condylartha are flat and nearly horizontal. 
In higher forms, especially of the ungulate series, they become 
curved, the posterior turning upwards and outwards, and the an- 
terior embracing them on the external side. In the higher Dip- 
lartha this curvature is followed by another curvature of the 
y ; 
