1885.] Progressive and Retrogressive. 145 
custom to define the classes by means of these characters, taken 
in connection with those of the skeleton. Commencing in the 
Leptocardii with the simple tube, we have two chambers in the 
Marsipobranchii and fishes; three in the Batrachia and Reptilia; 
and four in the Aves and Mammalia. The aorta:roots commence 
as numerous pairs of branchial arteries in the Leptocardii; we 
see seven in the Marsipobranchi, five in the fishes (with number 
reduced in some); four and three in Batrachia, where they gener- 
ally cease to perform branchial functions; two and one on each 
side in Reptilia; the right hand one in birds, and the left hand 
one in Mammalia. This order is clearly an ascending one 
throughout. It consists of first, a transition from adaptation to 
an aquatic to an aérial respiration ; and second, an increase in the 
power to aérate and distribute a circulating fluid of increased quan- 
tity, and of increased calorific capacity. In other words, the cir- 
culation passes from the cold to the hot-blooded type coinciden- 
tally with.the changes of structure above enumerated, The 
accession of a-capacity to maintain a fixed temperature while that 
of the surrounding medium changes, is an important advance in 
animal economy. 
The brain and nervous system also display a general progres- 
sive ascent. Leaving the brainless Leptocardii, the Marsipo- 
branchs and fishes present us with small hemispheres, larger 
optic lobes and well-developed cerebellum. The hemispheres are 
really larger than they appear to be, as Rabl Riickard has shown’ 
that the supposed hemispheres are only corpora striata. But 
the superior walls are membranous, and support on their in- 
ternal side only a layer of epithelial cells, as in the embryos of 
other Vertebrata, instead of the gray substance. So'that although 
we find that the cerebellum is really smaller in the Batrachia and 
most Reptilia than in the fishes, the better development of the 
hemispheres in the former gives them the preéminence. The 
Elasmobranchii show themselves superior to many of the fishes 
in the large size of their corpora restiformia and cerebellum. The 
Reptilia constitute an advance on the Batrachia. In the latter the 
optic thalami are, with some exceptions, of greater diameter than 
the hemispheres, while the reverse is generally true of the rep- 
tiles. The crocodiles display much superiority over the other 
1 Biologisches centralblatt, 1884, p- 449 
VOL, XIX.—NO, 1I. 
