CHAPTER XXII. THE THEOKY OF EVOLUTION. 



1. Relationship between Types. We have now con- 

 sidered our Vertebrate types, both from the standpoint of 

 adult anatomy and from embryological data ; and we have 

 seen through the series a common structure underlying wide 

 diversity in external appearance and detailed anatomy. We 

 have seen a certain intermediateness of structure in the 

 frog, as compared with the rabbit and dogfish, notably in 

 the skull and skeleton, in the circulation, in the ear, and in 

 the reduced myomeres ; and we have seen that the rabbit 

 passes in these respects, and in others, through dogfish- and 

 frog-like stages in its development, and this alone would be 

 quite sufficient to suggest that the similarities of structure 

 are due to other causes than a primordial adaptation to 

 certain conditions of life. 



2. Theory of Common Ancestry. The explanation of 

 these resemblances according to the theory of evolution is 

 that they are family likenesses in other words, the marks 

 of a common ancestry. The theory attributes the actual 

 structure of organisms at the present day to heredity with 

 modification. We have no scientific evidence of any other 

 origin for individual organisms than by the process of 

 reproduction from pre-existing individuals. We must 

 conclude that all Vertebrates are descended from the same 

 primitive type. 



It must not be imagined for a moment that rabbits are 

 supposed to be descended from frogs, frogs from dogfish, or 

 dogfish from lancelets. Adopting the customary likening 

 of genealogy to a tree, we may say that these and other 

 animal species now existing do not represent parts of the 

 trunk and branches of the trees, but only the terminal 

 leaves. Nevertheless their structure gives us clear evidence 

 of the approximate point where the branches which boar 

 them diverged from one another or from the trunk. 



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