304 EVOLUTION IN BIOLOGY. [LECT. 



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 com- 

 parison 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, morphology exhibits a continual 

 advance towards the demonstration of a fundamental 

 unity among the seeming diversities of living struc- 

 tures. 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 elementary 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 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 

 protoplasmic material. 



5. The innumerable cases of structures, which are 

 rudimentary and apparently useless, in species, the 

 close allies of which possess well developed and func- 

 tionally important homologous structures, are readily 

 intelligible on the theory of evolution, while it is hard 

 to conceive their raison d'etre on any other hypothesis. 

 However, a cautious reasoner will probably rather 



