CHAPTER XII. 



THE TYPES OF CONSTRUCTION IN THE ANIMAL 

 KINGDOM. 



Records of Evolution expressed chiefly in Generic and Specific 

 Characters. — From what has been said in the previous chapter 

 it will be learned that the grand features and the great 

 majority of the more important details of the structure of any- 

 living organism are of extreme antiquity. Not only so, but 

 since very early geological time no new types of structure of 

 as high as ordinal rank have been evolved in the majority of 

 the branches of the Animal Kingdom. 



In respect, therefore, to a great number of the more im- 

 portant characters of organisms the development of offspring 

 has resulted in the repetition, without substantial modifica- 

 tion, of the characters of the ancestors. This is the law of 

 Heredity — the repetition in the ofTspring by generation of 

 characters like those of its ancestors. Evolution has to do zvith 

 iJie acquirement by organisms of morphological characters zuhich 

 their ancestors did not possess ; hence we must seek for evidences 

 of evolution chiefly among the characters of less than ordinal 

 rank — those of ordinal and higher rank having been evolved 

 almost at the beginning of the history. 



Course of Individual Development supposed to have been Con- 

 stant. — It is not unreasonable to assume that all the course 

 and the stages of development, of characters of ordinal and 

 higher rank in the development of the individual, are repeti- 

 tions of what has taken place since their first appearance at 

 tne beginning of the geological time-record. In the several 

 types of organisms now living, the laws of individual devel- 

 opment, as of the steps by which in each case diversity is 

 elaborated out of simplicity of structure, may reasonably be 

 regarded as applicable to all organisms of which we can study 



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