220 EVOLUTION IN BIOLOGY vi 



which are closely analogous to those which are 

 permanent in some fishes. 



4. That branch of biology which is termed Mor- 

 phology is a commentary upon, and expansion of, 

 the proposition 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 o-f 

 the homologies of the parts of a flower by C. F. 

 Wolff, to the present elaborate analysis of the 

 floral organs, morphology exhibits a continual 

 advance towards the demonstration of a funda- 

 mental unity among the seeming diversities of 

 living structures. And this demonstration has 

 been completed by the final establishment of the 

 cell theory, which involves the admission of a 

 primitive conformity, not only of all the elemen- 

 tary structures in animals and plants respectively, 

 but of those in the one of these great divisions 

 of living things with those in the other. No a 

 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 

 protoplasmic material. 



5. The innumerable cases of structures, which are 



