THE LONG ROAD 



ceeding one another, as Cope says, in the relative 

 order of their zoological rank. Thus the sponges are 

 later than the protozoa, the corals succeed the 

 sponges, the sea-urchins come after the corals, the 

 shell-fish follow the sea-urchins, the articulates are 

 later than the shell-fish, the vertebrates are later 

 than the articulates. Among the former, the am- 

 phibian follows the fish, the reptile follows the am- 

 phibian, the mammal follows the reptile, and non- 

 placental mammals are followed by the placental. 



It almost seems as if nature hesitated whether to 

 produce the mammal from the reptile or from the 

 amphibian, as the mammal bears marks of both 

 in its anatomy, and which was the parent stem is 

 still a question. 



The heart started as a simple tube in the Lepto- 

 cardii ; it divides itself into two cavities in the 

 fishes, into three in the reptiles, and into four in the 

 birds and mammals. So the ossification of the ver- 

 tebral column takes place progressively, from the 

 Silurian to the middle Jurassic. 



The same ascending series of creation as a whole 

 is repeated in the inception and development of 

 every one of the higher animals to-day. Each one 

 begins as a single cell, which soon becomes a con- 

 geries of cells, which is followed by congeries of con- 

 geries of cells, till the highly complex structure of the 

 grown animal with all its intricate physiological 

 activities and specialization of parts, is reached. It 



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