EXISTING ORGANISMS— ANATOMY 89 



gross morphology. The resulting adaptations often appear to be 

 but a poor makeshift. They are effective, since their effectiveness 

 is necessary to existence of the species, but they seem crude in 

 comparison to organs of other animals which are used in the same 

 way. Enough examples can be observed among the many species 

 of animals of very different ways of attaining the same end effec- 

 tively, such as the wings of birds and insects, to make possible 

 only one interpretation of the existing conditions of resemblance 

 among vertebrates, and among invertebrates as well. We can 

 conclude only that the fundamental similarity of their often dif- 

 ferent structures in any system is evidence of definite relationship; 

 that the dolphin's flippers and the human hand have evidently 

 homologous structure not because such structure is the only 

 possible foundation for a swimming organ and a prehensile ap- 

 pendage but because their ancestors were related in possessing 

 just such a foundation as the pentadactyl appendage, of which 

 they have made different uses. 



Summary. The anatomy of adult animals of the vertebrate 

 classes shows many points of similarity. All parts of the skeleton 

 are evidently based on the same plan of structure. The differences 

 which appear are easily correlated with special habits. Exo- 

 skeletal structures of different kinds also show fundamental simi- 

 larity. Relationship of other systems is closely linked with 

 embryology, but the presence of vestigial organs is significant, 

 especially in man. Since these organs are useless or nearly so the 

 only possible explanation of their presence is that they are the 

 remains of once useful organs which the animal has not yet entirely 

 lost. 



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Reynolds, S. H., The Vertebrate Skeleton, 2nd edition, 1913. 



Walter, H. E., The Human Skeleton, 1918. 



Cunningham's Textbook of Anatomy, 5th edition, 1921. 



Newman, H. H., Readings in Evolution, Genetics and Eugenics, 1921. 



Wilder, H. H., History of the Human Body, revised edition, 1923. 



Kingsley, J. S., Comparative Anatomy of Vertebrates, 3rd edition, 1926. 



