A Story Outline of Evolution 



The nerve conductors of the lov^er forms gradually 

 wove themselves into a central string commonly known as 

 the notochord. Around this sensitive organ a cartilaginous 

 substance was formed which was the beginning of the back- 

 bone. The many jointed segments of the lower forms, 

 through adaptation became the skull and vertebrate of the 

 higher forms which were to follow. As necessity for a 

 larger brain developed, the segments which formed a part 

 of the invertebrate forms were pushed out and formed a 

 mould which surrounded as a protective covering the grow- 

 ing brain. The cartilaginous substance as a means of greater 

 protection through the course of ages became hardened 

 into bone. 



The gradation between the invertebrate and the verte- 

 brate animals, is the most obscure page in the history of life 

 but there was a class of animals, now long extinct, that con- 

 nect the two great divisions. Scientists call them ostraco- 

 derms and like the animals connecting one class with a higher 

 class, they possess some of the characteristics of both. They 

 were developing cartilage into bone. The first bone was 

 formed around the notochord which was the main channel 

 of sensation and this was formed in segments correspond- 

 ing with the segments of the body. As the vertebrate were 

 gradually formed, the protective armour that formed a 

 shield for the body was changed into more flexible forms 

 to correspond to the flexible vertebrate. This armour became 

 the scales of the fishes, the feathers of the birds and the 

 skin of the mammals. 



These ostracoderms were long and joined and had two 

 or three sets of jaws. They were a higher form than the 



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