Chap. VIII. FLOWEES OF ORCHIDS. 233 



can be most surely made out by tracing their embryo- 

 logical deyelopment when that is possible ; or by the 

 discovery of organs in a rudimentary condition ; or by 

 tracing, through a long series of beings, a close gradation 

 from one part to another, until the two parts or organs, 

 though employed for widely different functions and 

 most unlike each other, can be joined by a succession 

 of short links. No instance is known of a close gradation 

 between two organs, unless they are homologically one 

 and the same organ. 



The importance of the science of Homology rests on 

 its giving us the key-note of the possible amount of 

 difference in plan within any group ; it allows us to 

 class under proper heads the most diversified organs ; 

 it shows us gradations which would otherwise have 

 been overlooked, and thus aids us in classification ; it 

 explains many monstrosities ; it leads to the detection 

 of obscure and hidden parts, or mere vestiges of parts, 

 and shows us the meaning of rudiments. Besides 

 these uses. Homology clears away the mist from such 

 terms as the scheme of nature, ideal types, archetypal 

 patterns or ideas, &c. ; for these terms come to express 

 real facts. The naturalist," thus guided, sees that all 

 homologous parts or organs, however much they may 

 be diversified, are modifications of one and the same 

 ancestral organ ; in tracing existing gradations he 

 gains a clue in tracing, as far as that is possible, the 

 probable course of modification through which beings 

 have passed' during a long line of generations. He 

 may feel assured that, whether he follows embryo-^ 

 logical development, or searches for the merest rudi- 

 ment, or traces gradations between the most different 

 beings, he is pursuing the same object by different 

 routes, and is tending towards the knowledge of the 

 actual progenitor of the group, as it once grew and 



