EVOLUTION AS SEEN IN THE CLASSIFICATION OF ANIMALS 313 



similar in structure are classed together; animals dissimilar in structure are 

 separated. Characteristics used in classifying are the fundamental similari- 

 ties which we termed homologous in Chapter 3. Failure to distinguish 

 homologous similarity from analogous similarity (i.e., that connected with 

 similar function, pp. 20-26) results in errors of classification. Such an er- 

 ror was formerly made when whales were classified as fishes rather than 

 as mammals. 



To Linnaeus, two species which were similar in structure and therefore 

 to be classed in the same genus were not related to each other in any sense 

 of inheritance. In creating each separately the Creator had seen fit to make 

 them similar to each other, just as a carpenter may see fit to build two 

 houses which are much alike. The houses are not "related"; neither, to 

 Linnaeus and most of his contemporaries, were the species. 



Conceivably, then, fundamental similarities may be shared by two spe- 

 cies because these species were created to resemble each other, no genetic 

 relationship being present. On the other hand, fundamental similarities may 

 be shared by two species because these species were derived from a com- 

 mon ancestor, or because one species was the ancestor of the other. Com- 

 mon inheritance as the explanation for similarity accords with the evolu- 

 tionary interpretation, a matter we discussed at some length in Chapter 3 

 (see especially pp. 25-26). According to this view, a classification based 

 on fundamental similarities, as the Linnaean system is, becomes a classifi- 

 cation reflecting the actual relationships of the animals classified. The spe- 

 cies included in one genus are similar because they are related to each 

 other; they inherited their similarities from a common ancestor. Two gen- 

 era included in one family have many characteristics in common; these 

 were inherited from a predecessor ancestral to both genera. And so on step 

 by step through the classification. Creatures as diverse as a fish and a man 

 are included in one phylum, to pass to the "top" of our classification. Fish 

 and man have some characteristics in common, notably possession of a 

 notochord during some stage of life. Why do they and all other members of 

 Phylum Chordata have this notochord? If the evolutionary explanation is 

 correct they have it because they inherited it from an ancient form which 

 once lived on this planet and was the remote ancestor of all of them. 



It is customary today to refer to the classification of animals as a "natu- 

 ral system of classification." By this is meant a system based on the true 

 (i.e., genetic) relationships of the animals classified. A natural system may 

 be contrasted with an "artificial system" of classification having as its sole 

 objective the cataloguing of plants and animals as a librarian classifies and 

 catalogues books. The system as devised by Linnaeus was essentially an 



