VESTIGIAL STRUCTURES 127 



is born with a three-chambered heart, it is gratuitous to call this 

 a reversion to the saurians with three-chambered hearts, from which 

 mammals evolved. It is simply a case of arrested development. 



Vestigial Structures. It is a familiar fact that structures 

 of ancient origin and erstwhile importance may still linger in 

 dwindled expression in organisms where they do not seem to 

 have much or any significance. They are relics of the past, 

 vestiges of ancestral history, comparable, as Darwin said, to 

 the unsounded letters in many words, the o in leopard, or the 

 b in doubt non-functional vestigial letters of which the spelling- 

 reformers would rob us so ruthlessly. 



Each one of us is a walking museum of such relics, some of 

 which we should probably do better without. Thus the unused 

 muscles of the ear and the rudimentary third eyelid are ancestral 

 characters which persist in us, though without much significance 

 now. They are like the unused, often unusable, buttons, etc. 

 which survive on some parts of our every-day attire useless, 

 but interesting, vestiges of bygone days. The gill-clefts of reptiles, 

 birds, and mammals ; the embryonic teeth of whalebone whales ; 

 the buried remains of pelvis and hind-limbs in whales ; the hint 

 of 3. gill in the skate's spiracle, and so on, are familiar examples 

 of these " vestigial structures," traces of ancestral history, and 

 intelligible on no other theory. 



But it goes without saying that as the occurrence of these 

 vestigial structures is still normal, there is no utility in calling 

 them " reversions " even if now and again they are expressed 

 in greater strength than usual or persist beyond the time at 

 which many of them (e.g. all the gill-clefts save one) disappear 

 namely, during development. They are very interesting, how- 

 ever, (i) in showing that ancestral features have great power of 

 hereditary persistence, and (2) inasmuch as they often show 

 great variability. 



Acquired Modifications resembling Ancestral Characters. 

 When an individual exhibits a structural peculiarity not ex- 



