EVIDENCE FURNISHED BY SCIENCE OF LIKENESSES. 123 



The " science of likenesses " is known to specialists as " Ho- 

 mology," and it may further our ready appreciation of the details to be 

 presently treated in these pages if we make mention likewise of the 

 term " analogy " and its meaning. The latter word is, as a rule, very 

 loosely used in ordinary life. Scientifically employed, its meaning is 

 clear enough. In a dictionary we find it explained as meaning " cor- 

 respondence, or likenesses in some ways, proportions, or effects." 

 Obviously, the term is used in a general sense to mean any degree of 

 likeness, resemblance, or relationship between objects. In science, 

 the word "analogy" has but one distinct meaning. It implies 

 identity or correspondence in function or use, and nothing more. 

 When two things are used for the same purpose they are "analogous ;" 

 and no further resemblances or likenesses are required in science to 

 justify the use of the term. Every one knows that a bird's wing is a 

 very different structure from that of a fly or butterfly. The one is 

 really a fore-limb ; the other being merely an expansion of the skin of 

 the body. But despite their wide difference in structure, they are 

 truly " analogous," being used for one and the same purpose, that of 

 flight. In this sense alone, can any two objects be truly termed 

 " analogous." 



Now, turning to " homology," we discover a deeper relationship 

 between organs and parts than that indicated by analogy. That two 

 things may be truly named " homologous " it is not necessary to 

 think of their use in any sense. The all-important consideration on 

 which the science of likenesses hangs, is the fact of identity or 

 correspondence in fundamental stmctitre or in origin. Such a 

 correspondence is illustrated by the subject of limbs already referred 

 to. The arm of man, the fore-leg of a horse, and the wing of a bird, 

 are used each for a different purpose. They are not " analogous," but 

 they are undoubtedly " homologous," because, beneath the diversity of 

 form and function, we can readily perceive the striking similarity of 

 fundamental structure or type. Thus things may not be what they 

 seem, when viewed by homology for the wings of bird and butterfly, 

 alike in the popular sense, are utterly unlike ; and regarded in the 

 same light many things are what they do not seem. The seeming 

 unlikenesses of arm, wing, and fore-leg are thus merely superficial, 

 and serve to hide the deeper realities that link them firmly together 

 as the same in type, and presumably the same in origin. It may 

 happen, lastly, that two organs may be both analogous and homo- 

 logous. But the presence of both degrees of likenesses is at the 

 best accidental, or induced by like conditions of life which do not 

 affect the deeper considerations which homology brings before us. 

 The wing of the bird and that of the bat are formed each from a 

 fore-limb although in diverse ways and each subserves the purpose 

 of flight. Analogy and homology seem to run in parallel lines in this 



