28 " "THE COMING OF MAN 



eyes have the possibility of sight at long range. Smell and 

 hearing are keen. The first chordata seem to have lived 

 on minute food, sifting it out of the water with their gill-sack. 

 Then jaws were developed armed with rows of sharp and 

 pointed teeth. The sense-organs are innervated from a brain 

 containing, even in the most primitive forms, the nervous 

 material of some five segments, later of probably nine and 

 even more. Powerful muscles and a large brain characterize 

 this group from the earliest times. The revolution prophesied 

 in worms has been accomplished. They swim far and swiftly, 

 continually meeting new experiences and dangers throughout 

 the years of a long life. Surely some of their descendants 

 will become intelligent. 



Such changes take time and can come about only very 

 slowly. Millions of years must have elapsed before a form 

 somewhat like amphioxus could have changed even into the 

 most ancient shark. Cartilage had to be discovered, invented, 

 hit upon and improved — express it as you will. It could arise 

 only under strain in the fibers of the dorsal fin-rays; it 

 gradually extended downward on the arches over the nervous 

 chord, thence into the sheath of the notochord. We wonder 

 that the process was ever carried to a successful issue. They 

 looked like anything but promising forms. 



But replacing fibrous tissue by cartilage was only the first 

 step. The only bone in a primitive shark was in the minute 

 scales or denticles of the skin. How some ancient ancestor 

 had succeeded in developing bone, in many respects the most 

 marvellous tissue in our bodies, is beyond my comprehension 

 or imagination. He did it. 



Bone also worked its way downward from the surface along 

 the rays and spinal arches, as cartilage had preceded it; and 

 finally replaced the cartilage. In time vertebras were shaped 

 and the animal had a backbone. We might say that ver- 

 tebrates built three skeletons in order to gain the beginnings of 

 a backbone, a marvellous series of changes. 



We may glance at the structure of a shark as a very prim- 

 itive form of vertebrate animal. We notice the long cylin- 



