EVOLUTION IN BIOLOGY. 3H 



development, is an organism which, if it had an inde- 

 pendent existence, must be classified among fishes ; and 

 all the organs of the reptile pass, in the course of their 

 development, through conditions which are closely analo- 

 gous to those which are permanent in some fishes. 



4. That branch of biology which is termed Morphol- 

 ogy 'is a commentary upon, and expansion of, the propo- 

 sition that widely different animals or plants, and widely 

 different parts of animals or plants, are constructed upon 

 the same plan. From the rough comparison of the 

 skeleton of a bird with that of a man by Belon, in the 

 sixteenth century (to go no farther back), down to the 

 theory of the limbs and the theory of the skull at the 

 present day; or, from the first demonstration of the 

 homologies of the parts of a flower by C. F. Wolff, to 

 the present elaborate analysis of the floral organs, mor- 

 phology exhibits a continual advance towards the demon- 

 stration of a fundamental unity among the seeming diver- 

 sities of living structures. And this demonstration has 

 been completed by the final establishment of the cell 

 theory, which involves the admission of a primitive con- 

 formity, not only of all the elementary structures in ani- 

 mals and plants respectively, but of those in the one of 

 these great divisions of living things with those in the 

 other. No d priori difficulty can be said to stand in the 

 way of evolution, when it can be shown that all animals 

 and all plants proceed by modes of development, which 

 are similar in principle, from a fundamental protoplas- 

 mic material. 



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