30 HOMOLOGY. [IXTROD. 



this assumption is unfounded; torso far are the expressions of it 

 which are called Metamerism from standing alone, that it is 

 almost impossible to look at any animal or vegetable form without 

 meeting phenomena of Repetition which differ from Metamerism 

 onlv in degree or in extent. Between these Repetitions and 

 Metameric Repetitions it is impossible to draw any line, and the 

 Variations of all will therefore be treated together. 



This error in the estimate of the value of Metamerism as a 

 guide to phylogeny is one by which the evidence of Variation is 

 only indirectly affected. The other errors now to be mentioned 

 are of a much more serious nature, for they concern the general 

 conception of the nature of Homology which is the basis of all 

 morphological study. 



In introducing the method of the Study of Variation I have 

 said that it can alone supply a solid foundation for inquiry into 

 the manner by which one species arises from another. The facts 

 of Variation must therefore be the test of phylogenetic possibility. 

 Looking at organs instead of species, we shall now see that the 

 facts of Variation must also be the test of the way in which organ 

 arises from organ, and that thus Variation is the test of Homo- 

 logy. For the statement that an organ of one form is homologous 

 with an organ of another means that there is between the two 

 some connexion of Descent, and that the one organ has been 

 formed by modification of the other, or both by modification of a 

 third. The precise way in which this connexion exists is not 

 defined, and indeed has scarcely ever been considered, though 

 such a consideration must sooner or later be attempted. We 

 must for the present be content with the belief that in some un- 

 defined way there is a relationship between ' homologous ' parts, 

 and that this is what we mean when we affirm that they are 

 homologous. 



We have however assumed that the transition from one form 

 to another takes place by Variation. If therefore we can see the 

 variations we shall see the precise mode by which the descent is 

 effected, and this must be true of the parts or organs as it is true 

 of the whole body. In like manner then as the Study of Variation 

 may be hoped to shew the way by which one form passes into 

 another, so also may it be hoped that it will shew how the organs 

 of one form take on the shape of the homologous organs of 

 another. 



In the absence of the evidence of Variation reasoning as 

 to Homology rests solely on conjecture, and assumptions have 

 thus been made respecting the nature of Homology which have 

 coloured the whole of morphological study. Of these, two demand 

 attention now. 



I. As to Homology between the Members of one Series. We 

 saw above (page 29) how the resemblance between Repetitions 



