262 LESSONS FROM NATUEE. [CHAP. VIII. 



animals, and show more and more plainly the entangled inter- 

 dependencies of their structure. The notion, once popular 

 with Evolutionists, that &quot; similarity of structure &quot; necessarily 

 implies &quot; genetic affinity,&quot; can certainly now be maintained, 

 as a biological axiom, by no well-informed naturalist. 

 ~ Indeed, the distinction between homogeny and homoplasy 

 (between the influence of a common descent and that which 

 produced independent similarity) has its importance mucli 

 reduced through the power which the latter possesses of 

 simulating the former. The degree to which homoplasy 

 can rival homogeny in the degree of likeness produced, is 

 shown, not only by the instances cited, but also by the 

 likenesses existing between some of the bones of the skull 

 in beasts and in osseous fishes. Probably but few naturalists 

 would now dispute the independent origin of the bones of 

 the skull in these two classes of animals. Yet their cranial 

 bones are in many instances indisputably homologous, while 

 in others their homology is a subject of keen discussion. 



If it be asked what is meant by parts being &quot; homolo 

 gous/ if they are not &quot;homogenetic,&quot; it may be replied 

 that it means they show a complex likeness, or agreement, 

 as to their relative positions to other surrounding parts. 

 This likeness, or agreement, may be of different kinds, ac 

 cording as we follow different lines of thought. An intellect 

 of a higher order than that of man would probably detect 

 an indefinite number of relations between two animals and 

 between their component parts, which relations escape our 

 observation altogether, though we can readily enough ap 

 prehend a considerable number of such relations. 



Thus we may enumerate as examples of different kinds of 



Catalogue of homology I- 



homologies. } p artg w } 1 j cn nave ft S i m il ar ity O f function but 



differ structurally in their relations to all the rest of the 

 body (i.e., differ in their relative position to the rest of the 

 body) e.g., the legs of a lizard and of a lobster. 



2. Parts which are similar both as to function and relative 

 position e.g., the wings of a bat and of a bird. 



