CHAPTER TWO 



Evidences of Evolution 

 I: Biop'eo^rahh 



BiOGEOGRAPHY, taxonomy, physiology, comparative anatomy, embryology, 

 and paleontology have usually been presented as the fields from which the 

 evidence of evolution is primarily derived. Some of the more important 

 evidence from each of these fields will be summarized in this and the fol- 

 lowing chapters. However, genetics must be regarded as a seventh field 

 of evidence for evolution, for a large part of the current literature on 

 evolution is drawn from this field. 



The study of biogeography, or the geographical distribution of plants 

 and animals, is of particular interest, because this is the field which first 

 directed Darwin's attention while on the Beagle to the possibility of the 

 origin of species by means of evolution. He regarded the voyage of the 

 Beagle as the most important event in his life. In his autobiography he 

 states: "During the voyage ... I had been deeply impressed by discover- 

 ing in the Pampean formation great fossil animals covered with armour 

 like that on the existing armadillos; secondly, by the manner in which 

 closely allied animals replace one another in proceeding southwards over 

 the continent; and thirdly, by the South American character of most of 

 the productions of the Galapagos Archipelago, and more especially by the 

 manner in which they differ slightly on each island of the group; none of 

 the islands appearing to be very ancient in a geological sense. 



"It was evident that such facts as these, as well as many others, could 

 only be explained on the supposition that species become modified; and 

 the subject haunted me. But it was equally evident that neither the action 

 of the surrounding conditions, nor the will of the organisms (especially 

 in the case of plants) could account for the innumerable cases in which 

 organisms of every kind are beautifully adapted to their habits of life— 

 for instance, a woodpecker or a tree frog for climbing trees, or a seed for 

 dispersal by hooks or plumes. I had always been much struck by such 

 adaptations, and until these could be explained it seemed to me almost 

 useless to endeavour to prove by indirect evidence that species have been 

 modified. . . . 



"In October 1838, that is, fifteen months after I had begun my system- 



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