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frequent in most groups of animals and higher plants and that 

 the frequency and significance of their occurrence can be deter- 

 mined rather accurately; (c) that such difficulties are usually of 

 minor importance in nondimensional situations which are those 

 most frequently encountered by the taxonomist; and (d) that in 

 spite of these difficulties it is usually possible to classify doubtful 

 entities into taxonomic species which satisfy at least one or the 

 other species concept. To use the words of G. G. Simpson ( 1943) : 

 "A taxonomic species is an inference as to the most probable 

 characters and limits of the morphological species from which a 

 given series of specimens is drawn." 



The Practical Importance of Species 



Those who maintain that species are something purely sub- 

 jective, vague, and arbitrary sometimes ask: "What is gained by 

 recognizing species?" The answer is that the attempt to determine 

 species status has led in many cases not only to a more precise 

 formulation of a biological problem but very often also to its 

 solution. As an illustration I will call attention to only three areas 

 of biological research where this is true. 



The Clarification and Simplification of Classification. The 

 need for classifying the morphological diversity of nature into 

 biological species forces an unequivocal decision how to handle 

 morphological variants. Accepting the biological species concept 

 no longer permits either describing all variants as morphological 

 species or listing the more pronounced ones as species and the 

 less distinct ones as varieties. Now only those variants are given 

 species rank which satisfy the criterion of the biological species 

 concept, namely, reproductive isolation. The result is a simplifica- 

 tion of classification which is not only of practical help to the 

 working taxonomist but also actually aids the understanding of 

 distribution, ecology, and phylogeny. In ornithology it has per- 

 mitted a reduction in the number of species from nearly 20,000 

 to 8,600, and a similar simplification is apparent throughout zool- 

 ogy. There are still some authors who resent having to make 



