ATAVISM OR REVERSION. 137 
In the embryo these flaps, or valves, arise as outgrowths 
from the cardiac walls, and consist of muscle tissue, 
which gradually undergoes metamorphosis into fibrous 
tissue. In the echidna and ornithorhynchus the valve 
of the right ventricle is muscular throughout life : this is 
also the case in all birds. Hence a patch of muscle 
tissue in the cardiac valves may be regarded as atavistic. 
The arch of the aorta, the main arterial trunk, now 
and then illustrates this form of atavism. In all mam- 
mals the aorta in its normal course should curve over the 
root of the left lung; 
¢; in birds, with equal constancy, it 
curves over the root of the right lung; in lizards and 
frogs an aortic arch crosses the root of each lung and, 
subsequently uniting, form a single trunk. In the em- 
bryo of birds and mammals both aortic arches exist, but 
in mammals the right one becomes suppressed ; occa- 
sionally, however, the left arch, from causes quite 
unknown to us, undergoes suppression, the right one 
becoming the functional vessel. Thus a right aortic 
arch in a mammal is a true instance of atavism. 
It is well known that in the small amount of hair 
upon the body, man differs from all mammals except 
the whales and porpoises. He however stands alone in 
its unusual distribution. The human foetus is covered 
with delicate hair known as lanugo, which is usually 
shed shortly before or immediately after birth. This is 
clearly an indication of a lost character, the lanugo 
representing the hairy covering of closely allied verte- 
brates; it appears in the embryo in obedience to 
heredity. Children are often born with pigmented hairy 
patches on their bodies known as moles ; these moles 
