CHAPTER XX. 



COMPAEATIVE MOEPHOLOGY. 



TN the last chapter we have come to a conclusion which 

 -*- may be thus briefly recapitulated. Although the law 

 of the adaptation of every part of an organism to the rest, 

 and of the whole organism to its mode of life, is true ; yet 

 there are many facts of the morphology of single species 

 which this law will not account for, and which point 

 rather to a principle of correlation of form, analogous to 

 the formative law of crystallization. 



As yet I have spoken only of the morphology of Specific 

 single species. It is obviously the logical order thus ™gyPjj,gi. 

 to speak of the morphology of single species before the cally comes 

 comparative morphology of different species and classes ; compara- 

 tor, were there only one organic species in existence — *^^''" 

 whether man or the oak-tree, or any other species that has 

 a definite form — its morphology would be an object of 

 science; but it would be impossible to study the com- 

 parative morphology of different species and classes, without 

 basing the study on our knowledge of the morphology of 

 single species. 



Having arrived at the above-stated conclusion with 

 respect to the principles of what may be called specific 

 morphology, we shall find the facts of comparative mor- 

 phology support that conclusion, and lead to other kindred 

 results. 



It is necessary at the commencement to have a clear 

 conception of the difference between analogiccd relations Analogy 

 and homological relations. Two organs are analogous which w'^"'"'^* 

 perform the same function : for instance, the wing of the 



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