xii PREFACE. 



subserviency, to the completion of the most perfect act of that 

 function, the conversion, namely, of grass into flesh. 



Thus, in tracing through the animal series this course of parts 

 and organs, we pass from the many and the like to the few and 

 the diverse. 



A ' homologue ' is a part or organ in one organism so answer- 

 ing to that in another as to require the same name. 



Prior to 1843 the term had been in use, but vaguely or 

 wrongly. 1 * Analogue ' and 'analogy' were more commonly 

 current in anatomical works to signify what is now definitely 

 meant by ' homology.' But ' analogy ' strictly signifies the 

 resemblance of two things in their relation to a third ; it im- 

 plies a likeness of ratios. An ' analogue ' is a part or organ in 

 one animal which has the same function as a part or organ in 

 another animal. A e homologue ' is the same part or organ in 

 different animals under every variety of form and function. 



In the Draco volans (Vol. I. fig. 163) the fore-limbs are 

 'homologous' with the wings of the bird (Vol. II. fig. 1); the 

 parachute is ' analogous ' to them. 



Relations of homology -are of three kinds; the first is that 

 above defined. When the ' basilar process of the occipital bone ' 

 in Man is shown to answer to the distinct bone called ' basioc- 

 cipital ' in the fish, the special homology of that anthropotomical 

 process is determined ; as such homologies are multiplied, the 

 evidence grows that man and fish are constructed on a common 

 type. 



A wider relation of homology is that in which a part or series 

 of parts stands to such general type. When the ' basilar process 

 of the occipital bone ' is determined to be the ' centrum ' of the 

 last cranial vertebra, its general homology is enunciated. 



The archetype skeleton represents the idea of a series of 

 essentially similar segments succeeding each other in the axis 



1 ' Les organes des sens sont homologues, conime s'exprimerait la philosophic 

 allemande ; c'est-a-dire, qu'ils sont analogues dans leur mode de developpenient.' — 

 Geoffrey St. Hilaire, Annates des Sciencts Nat., torn. xii. 1825, p. 341. 



