76 THE GENESIS OF SPECIES. [CHAP. 



CHAPTER IH. 



THE COEXISTENCE OF CLOSELY-SIMILAR STRUCTURES OF 

 DIVERSE ORIGIN. 



Chances against Concordant Variations. Examples of Discordant Ones. Concordant 

 Variations not unlikely on a non -Darwinian Evolutionary Hypothesis. Placental 

 and Implacental Mammals. Birds and Eeptiles. Independent Origins of Similar 

 Sense Organs. The Ear. The Eye. Other Coincidences. Causes besides Natural 

 Selection produce Concordant Variations in Certain Geographical Regions. Causes 

 besides Natural Selection produce Concordant Variations in Certajn Zoological and 

 Botanical Groups. There are Homologous Parts not genetically related. Harmony 

 in respect of the Organic and Inorganic Worlds. Summary and Conclusion. 







THE theory of " Natural Selection " supposes that the 

 varied forms and structure of animals and plants have been 

 built up merely by indefinite, fortuitous, 1 minute variations 

 in every part and in all directions those variations only 

 being preserved which are directly or indirectly useful to 

 the individual possessing them, or necessarily correlated 

 with such useful variations. 



On this theory the chances are almost infinitely great 

 against the independent, accidental occurrence and pres- 

 ervation of two similar series of minute variations result- 

 ing in the independent development of two closely-similar 

 forms. In all cases, no doubt (on this same theory), some 

 adaptation to habit or need would gradually be evolved, 

 but that adaptation would surely be arrived at by different 

 roads. The organic world supplies us with multitudes of 



1 By accidental variations Mr. Darwin does not, of course, mean to 

 imply variations really due to " chance," bat to utterly indeterminate 

 antecedents. 



