162 The Outline of Science 



namely the Lancelets and the Round-mouths (Cyclostomes), 

 such as the Lamprey. They have no backbone in the strict sense, 

 but they have this notochord. It can easily be dissected out in 

 the lamprey a long gristly rod. It is surrounded by a sheath 

 which becomes the backbone of most fishes and of all higher ani- 

 mals. The interesting point is that although the notochord is 

 only a vestige in the adults of these types, it is never absent from 

 the embryo. It occurs even in man, a short-lived relic of the 

 primeval supporting axis of the body. It comes and then it goes, 

 leaving only minute traces in the adult. We cannot say that it 

 is of any use, unless it serves as a stimulus to the development of 

 its substitute, the backbone. It is only a piece of preliminary 

 scaffolding, but there is no more eloquent instance of the living 

 hand of the past. 



One other instance must suffice of what Professor Lull calls 

 the wonderful changes wrought in the dark of the ante-natal 

 period, which recapitulate in rapid abbreviation the great evolu- 

 tionary steps Which were taken by man's ancestors "during the 

 long night of the geological past." On the sides of the neck of 

 the human embryo there are four pairs of slits, the "visceral 

 clefts," openings from the beginning of the food-canals to the sur- 

 face. There is no doubt as to their significance. They corre- 

 spond to the gill-slits of fishes and tadpoles. Yet in reptiles, 

 birds, and mammals they have no connection with breathing, 

 which is their function in fishes and amphibians. Indeed, they are 

 not of any use at all, except that the first becomes the Eustachian 

 tube bringing the ear-passage into connection with the back 

 of the mouth, and that the second and third have to do 

 with the development of a curious organ called the thymus 

 gland. Persistent, nevertheless, these gill-slits are, recalling 

 even in man an aquatic ancestry of many millions of years 

 ago. 



When all these lines of evidence are considered, they are seen 

 to converge in the conclusion that man is derived from a simian 



