2124 A TEXTBOOK OF THEORETICAL BOTANY 



on biochemical tests, by which morphologically identical types may be 

 discriminated. The biochemical exploration of higher plants has only been 

 carried out to a very limited extent, but instances are known of " chemical 

 species " even among the Angiosperms. A good example is Eucalyptus 

 dives, the oil of which contains piperitone, cineole and phellandrine. Four 

 types of tree are known, all included in the same morphological species, 

 in which the relative proportions of these constituents are markedly and 

 constantly different, so that collections of the oil are mixed from trees of 

 known types in order to keep the product reasonably uniform. Another 

 similar case is that of Pinus zvashoemis from the western United States, where 

 different trees, morphologically identical, yield oils which are either 

 laevorotatory and contain a large percentage of carene, or dextrorotatory, 

 with carene replaced by fi-pinene. 



These differences are permanent and are hereditary. The different races 

 are therefore natural species within the terms of Darlington's definition, 

 and if they are assigned to a lower rank it can only be on grounds of 



convenience. 



Opinions differ as to the treatment of such physiological variations in 

 current systematics. Some workers prefer to limit specific rank to types 

 which are morphologically distinct, both among the flowering plants and 

 among the biologic races of the Fungi. Others take a broader view and 

 accord them specific rank. Among the Bacteria, at least, this is almost 



inevitable. 



Cytological " species " are probably common, but they have been 

 uncovered only in a limited number of cases. Such are, for example, 

 polyploid strains within a morphological species, which usually fail to cross 

 with the diploid strains and are thus genetically isolated. Another type 

 arises from inversions of part of a chromosome. The genie combination in 

 the inverted portion is preserved intact because it cannot take part in cross- 

 ing-over at meiosis and if mutations accumulate in the inversion they may 

 form a basis of major discontinuity within the morphological species. They 

 may not always affect the external form, but, if they cause physiological 

 differences which are differently affected, for instance, by temperature, 

 they may be subject to ecological selection and hence lead to different 

 habitat preferences or a different geographical distribution and thus to 

 genetic isolation. Such hidden specific differences were first found in 

 Drosophila and led to the recognition of three cytological species, D. 

 simulans, D. pseudo-obscura and D. miranda, the two latter being inter- 

 sterile and therefore genetically isolated. 



All the above cases come under the general title of " Cryptic Species". 

 Unless some solution to the taxonomic dilemma can be found they are a 

 standing challenge to museum systematists. 



To point out these limitations does not imply a criticism of the morpho- 

 logical standpoint nor is it derogatory to the great work done by systema- 

 tists who have worked from that standpoint. It only shows that, as the 

 origins of species are multifarious, so their distinctions are manifold and not 



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