Lecture XXX. 273 



matter how divergent in form, as derived from a common ancestor or 

 stock, then the homologues find their natural explanation as being 

 modifications of the same part or structure of the ancestral stock. 

 Thus the numerous and close homologues in Funaria hygrometrica 

 and Marchantia polymorpha are strong evidence that these two 

 species, though now very different from one another, are descended 

 from a common stock. Where the homology is not so striking 

 but is still easily recognisable, as for example between Marchantia 

 polymorpha arid Pinus silvestris, we must still conclude that the two 

 have had a common ancestry, but evidently the much greater 

 difference shows that the divergence of the two lines of descent 

 must be attributed to the still more remote past. The existence of 

 homologous parts then shows the genetic relationship of classes and 

 divisions of species much in the same way as the existence of 

 modified parts establishes that relationship between comparatively 

 closely similar species. 



(d) The argument from Vestigial structures. Many instances 

 are known of parts and structures of organisms which have no as- 

 signable function, and, in some cases, may be even disadvantageous 

 to the individual. As an example of these we may cite the reduced 

 or fugitive vegetative cells of the pollen-grain of the Seed-plants, 

 the antipodal cells of the female gametophyte of the Angiosperms, 

 the fruit of the banana which never produces good seed, the ligule 

 of Selaginella and the classical example from the animal kingdom 

 the appendix of Man. On any theory of special creation it is 

 incredible that organisms would be endowed with these useless, or 

 even redundant parts, while if we regard existing plants and 

 animals as modifications and transformations of pre-existing forms 

 then it is natural that in these transformations the functions of 

 some parts will be usurped or entirely superseded by others. But 

 the parts themselves, still inherited from the ancestors in which 

 they were functional, are now functionless and reduced, being en- 

 croached upon by new developments. They are aptly compared 

 to the footsteps of a receding past gradually undergoing oblitera- 

 tion in the changing structure of the species. 



(2) Evidence from Distribution. 



The theory of Evolution furnishes a key for the explanation of 

 the most various morphological facts, and the more diverse the 

 facts it fits in with, the more convincing does the theory become. 

 When the range of facts explained by the theory is as large as has 

 been indicated the conviction of the truth of the theory grows 

 almost ' to a certainty. At the same time more direct proof would 

 be acceptable. If we could isolate a number of species and watch 



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