and with five toes, have already appeared. The development of the
leg out of the fin is one of the most difficult and least understood
problems of vertebrate comparative anatomy. The legs are at first
weak and scarcely capable of supporting the body. Only gradually do
they strengthen into the fore- and hind-legs of mammals, or into the
legs and wings of birds and old flying reptiles.

3. Changes in the circulatory and respiratory systems. The fish
lives altogether in the water and breathes by gills, but the dipnoi
among fishes breathes by lungs as well as gills. As long as
respiration takes place by gills alone, the circulation is simple;
the blood flows from the heart to the gills, and thence directly all
over the body; the oxygenated blood from the gills does not return
directly to the heart. But the blood from the lungs does return to
the heart; and there at first mixes in the ventricle with the impure
blood which has returned from the rest of the body. Gradually a
partition arises in the ventricle, dividing it into a right and left
half. Thus the two circulations of the venous blood to the lungs,
and of the oxygenated blood over the body, are more and more
separated until, in higher reptiles, they become entirely distinct.

As the animal came on land and breathed the air, more completely
oxygenated blood was carried to the organs, and their activity was
greatly heightened. As more and more heat was produced by the
combustion in muscular and nervous tissues, and less was lost by
conduction, the temperature of the body rose, and in birds and
mammals becomes constant several degrees above the highest summer
temperature of the surrounding air.

The changes in the brain affect mainly the large and small brain.