TAXONOMY, ANATOMY, EMBRYOLOGY 



status in zoology (as determined by the International Congress of Zo- 

 ology). 



T. H. Huxley has said, "That it is possible to arrange all the varied 

 forms of animals into groups, having this sort of singular subordination 

 one to the other, is a most remarkable circumstance." It is indeed. Lin- 

 naeus accounted for this by the theory of archetypes, which assumes that 

 the Creator worked from a series of plans, the archetypes, which was lim- 

 ited in number. These archetypes, like the plans in an architect's folio, 

 were not all equally distinct, but fell into definite, classifiable categories. 

 Thus each class would correspond to a major archetype, the various orders 

 within a class would be lesser archetypes, and so on down the hierarchy. 

 Thus, Linnaeus attributed the similaritv between species of a genus not 

 to descent from a common ancestor, but to the supposed fact that each is 

 a more or less imperfect copy of rather similar archetypes, plans of the 

 Creator. 



SIGNIFICANCE OF THE TAXONOMIC HIERARCHY 



Darwin's explanation of this "remarkable circumstance" is quite different. 

 The taxonomic categories simply represent degrees of blood relationship. 

 Thus, all members of the phylum Chordata have common ancestors, but 

 they are exceedingly remote, and hence only the most fundamental chor- 

 date characters are held in common by extreme members of the phylum. 

 Within any class, however, the degree of relationship is much closer, and 

 hence more numerous and less fundamental characters are held in com- 

 mon by diverse members of a class. All birds, for example, share many 

 characters in common. As one goes down the taxonomic scale this trend 

 becomes stronger until finally members of a single species differ only in 

 minor characters, and this because of their common inheritance. It is diffi- 

 cult to study any group of organisms in detail without feeling that this 

 argument is a cogent one. 



The Tree of Life. Taxonomists have always tried to summarize their 

 studies with diagrams. A tree is the most successful type of diagram, but 

 this has not always been evident. Linnaeus experimented with map-like 

 diagrams, but it was found that no arrangement was possible which would 

 always place similar forms together and always separate dissimilar forms. 

 Later, Lamarck and others tried to arrange living forms on a ladder-like 

 diagram, on the principle that the adaptation of living forms is progres- 

 sive, so that any particular animal should be preceded by one somewhat 

 lower in the scale of life and followed by another one somewhat higher. 

 In a general way, this can be done. It may be easily conceded that am- 

 phibians are more advanced than fishes, and that reptiles are more ad- 

 vanced than amphibians. But one cannot continue this series— birds, then 

 mammals. For, while one mammal has advanced so far beyond all other 

 animals that he alone studies the world in which he lives, nonetheless the 

 majoritv of birds are in every respect quite as "high" as are the majority 

 of mammals. And so it appears that two rungs are required at the same 

 level on the ladder. This type of dilemma is not only repeated frequently 



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