INTRODUCTION TO CRYPTOGAMIC BOTANY. 41 



tioiis of organised beings ; and we may, therefore, define 

 analogy to be resemblance of function ; homology, corres- 

 pondence of structure or origin.* 



27. The word homology, it will be seen then, is more confined 

 in its use than analogy ; for we can scarcely limit the latter word 

 within the bounds just prescribed, except where it is the correla- 

 tive of homology. Homology relates, for the most part, to organs; 

 analogy to organs and to organisms. Now resemblances of very 

 different kinds may exist between such organisms. They may be 

 similar to each other in many respects, and yet there may be no 

 positive relationship between them. Organs may be homologous 

 with perfect organisms, as the tissues of plants with some of the 

 lower Cryptogams ; but there can be only remote afiinity be- 

 tween them. There can be no intimate affinity where the one 

 class, order, or species, does not approach, in nature, close to an- 

 other ; there must be an identity in the most essential characters, 

 or such a gradual melting of the one into the other, as makes it 

 difficult to say where the distinction ends and begins. Lichens, 

 for instance, so gradually pass into fungi, that it is almost im- 

 possible to say to which division certain species belong. In such 

 a case it would be foolish to deny that there is a very close re- 

 lationship. The resemblance is not one of mere analogy. On 

 the other hand, though there is an intimate homology, up to a 

 certain point, between the several organs of which a lily is 

 composed, and a water lily, there is no affinity between them, 

 except so far as they are both members of the Phtenogamic 

 class. There is homology -f- of organs ; there is analogy in the 

 one plant, as a whole, compared with the other, but there is 

 no affinity. These principles will be found of great conse- 

 quence in the comparison which we shall shortly have to 

 institute between certain Cryptogams and Pha^nogams. 



28. Analogy, then, indicates resemblance between objects 

 which have not necessarily any close affinity ; such resemblance 

 may be of greater or less importance, but it is always liable 

 * See Carpenter's Principles of Comparative Physiology, Ed. 4, p. 6. 

 t Perhaps the nearest synonym to homologous is correspondent. But 

 as bodies may correspond, Loth in function and structure, it is obvious 

 that the word is not sufficient. 



