ATA VISM OR RE VERSION. 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 ancl, 

 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 



