170 THE GENESIS OF SPECIES. [CHAP. 



Now, it is here contended that the relationships borne 

 one to another, by various component parts, imply the exist- 

 ence of some innate, internal condition, conveniently spoken 

 of as a power or tendency, which is quite as mysterious as is 

 any innate condition, power, or tendency, resulting in the 

 orderly evolution of successive specific manifestations. 

 These relationships, as also this developmental power, will 

 doubtless, in a certain sense, be somewhat further explained 

 as science advances. But the result will be merely a 

 shifting of the inexplicability a point backward, by the 

 intercalation of another step between the action of the 

 internal condition or power and its external result. In the 

 mean time, even if by " Natural Selection " we could elimi- 

 nate the puzzles of the "origin of species," yet other 

 phenomena, not less remarkable (namely, those noticed in 

 this chapter), would still remain unexplained and as yet 

 inexplicable. It is not improbable that, could we arrive at 

 the causes conditioning all the complex inter-relations 

 between the several parts of one animal, we should at the 

 same time obtain the key to unlock the secrets of specific 

 origination. 



It is desirable, then, to see what facts there are in 

 animal organization which point to innate conditions 

 (powers and tendencies), as yet unexplained, and upon which 

 the theory of " Natural Selection " is unable to throw any 

 explanatory light. 



The facts to be considered are the phenomena of 

 " homology," and especially of serial, bilateral, and vertical 

 homology. 



The word " homology " indicates such a relation between 

 two parts that they may be said in some sense to be " the 

 same," or at least "of similar nature." This similarity, 

 however, does not relate to the use to which parts are put, 

 but only to their relative position with regard to other parts, 

 or to their mode of origin. There are many kinds of homol- 



