Lefevre — The Advance of Zoology in the Nineteenth Century. 79 



a single bone or claw of an extinct animal the entire body 

 might be reconstructed, so definite is the relation of one part 

 to another. 



Of still greater importance was the recognition of the fact 

 that structures of different animals or plants are frequently 

 built upon a similar plan, exhibiting thus a fundamental re- 

 semblance, even though the functions of the parts in question 

 may be quite different. This led to the recognition that 

 structure, not function, determines resemblance, for it was 

 discovered that organs practically identical in form and struc- 

 ture may be used for totally different purposes. There con- 

 sequently arose the important conception of homology and 

 analogy of parts, and organs possessing the same plan of 

 structure and general relations were said to be homologous, as 

 the wing of the bird and the fore-leg of the mammal, or the 

 lung of the higher vertebrates and the swim-bladder of the 

 fish; and organs differing in plan of structure, though having 

 a similar function, were regarded as being analogous, as the 

 wing of the bird and the wing of the insect. The principle 

 of homology, though its meaning was not understood at the 

 time, was destined to assume the highest prominence later on 

 when the fact had become established that structural resem- 

 blances of parts are due to a community of descent. 



One of the most noteworthy of the early homologies ad- 

 vanced at this period is that proposed by Goethe in his 

 *' Metamorphosis of Plants,'' published in 1790, in which he 

 maintained that the parts of a flower, sepals, petals, stamens 

 and pistil, though apparently widely different, are in reality 

 modified leaves; an homology which is still adhered to. 



The vertebral theory of the skull, independently put for- 

 ward by Goethe and Oken, should also be mentioned, for al- 

 though it has had to be discarded, it played an important part 

 in the development of the conception of homologies. Ac- 

 cording to the theory the skull of a vertebrate was supposed 

 to consist of a number of closely united segments, each rep- 

 resenting a modified vertebra, similar in all essential respects 

 to a single vertebra of the spinal column ; the skull would 

 therefore, merely represent the consolidated anterior region 

 of the back-bone. Later investigation of the skull of the 

 lower vetebrates, where it consists, not of separate parts more 



