248 



HABIT AND INTELLIGENCE. 



[chap. 



Homology 

 due to 

 common 

 descent. 

 Exceptions 

 due to 

 sponta- 

 neous 

 variation. 



Eudi- 



mentary 



organs. 



Toes of 

 Ungiilata. 



Leg-bones 

 of 



serpents : 

 ■wing- 

 bones of 

 apteryx. 



Compa- 

 rison of 

 these to 

 fossils. 



single ancestor. All those homological resemblances which 

 are evidently not due to adaptation are in my belief due to 

 community of descent. And those changes which are not 

 due to adaptation, such as the abnormal number of cervical 

 vertebrae in the sloth and the manati, are, I believe, cases of 

 spontaneous variation. 



What is more than any other set of facts impossible to 

 reconcile with the independent creation of every species, 

 or indeed with any theory of the origin of species, except 

 that of descent with modification, is the existence of rudi- 

 mentary or aborted organs. To the fact of the existence 

 of such organs I have alluded in a former chapter, as veiy 

 remarkable exceptions to the general law that all structure 

 is adapted to function. It is impossible to assign any 

 function for such parts as the rudimentary toes of some 

 Ungulata (hoofed animals), which appear as if they were 

 made for no purpose except to complete the number to 

 five, being the usual number of toes among quadrupeds ; 

 or for the rudimentary leg-bones of some serpents ; or for 

 the rudimentary wing-bones of the apteryx, a bird of New 

 Zealand, which has no external wings. The discovery of 

 rudimentary organs occupies a similar place in the history 

 and in the philosophy of biology that the discovery of the 

 general prevalence of fossils occupies in the history and 

 philosophy of geology. The old notion that the earth was 

 at once created as we see it would be difficult enough to 

 reconcile with the evident marks of aqueous and igneous 

 action during past time ; but it became obviously un- 

 tenable and absurd when it was discovered that the crust 

 of the earth is full of the buried shells and bones of 

 extinct races ; for it is impossible to believe that an Intel- 

 ligent Creator would create dead shells and bones ; and, in 

 my opinion, it is not less absurd to think that an Intel- 

 ligent Creator would create animals with useless organs.^ 



1 It has been suggested that the purpose of apparently useless parts may 

 be similar to that of excretory organs, and may consist in disposing of 

 useless matter. This, however, is totally inconsistent with the facts of 

 the case. Phosphate of lime, which is the mineral constituent of bone, is 

 not a material of which growing animals are likely to have too much. 

 (See Darwin's Origin of Species, p. 538.) 



