WHA T IS A SPECIES ? 933 



This surely indicates that the p re-Darwinian systematists, however incorrect 

 may have been the definitions of a species framed by them, did not essentially 

 differ from Darwin as regards what they, in actual practice, recognized as a 

 species. Is it not possible that both Earwin and one of the earlier systematists, 

 when considering whether two forms should be ranked as distinct species or as 

 varieties, might finally have solved the problem in essentially the same way, 

 viz., by considering whether, or not, the differences between them were such 

 as experience had proved, or as might by analogy be inferred, to occur among 

 the immediate offspring of a single individual ? 



Do not the above considerations force us to the conclusion that a species is 

 an actual entity, the character of which is independent of any theory of 

 evolution ? 



That this is so can scarcely be more strikingly demonstrated than by the 

 fact that both Darwin and De Vries recognize the Linnean conception of a 

 species as a practical unit for natural history classification. Seeing that the 

 vast majority of what are universally accepted as species are now, in nature, 

 found to be fixed and constant, just as was the case in the time of Linnaeus, 

 this is no more than we should be naturally led to expect. 



Was it not with the object of emphasizing this all-important fact that 

 Linnaeus, in his famous definition, describes, species as remaining unchanged 

 since they were created by the " Infinitum Ens " ? 



But by thus emphasizing the characteristic fixity of species he obviously 

 attempted, not only to define a species, but to define as well how and when 

 each species originated — an altogether different thing. 



7. It is to be regretted that many modern definitions appear to rather in- 

 crease than to diminish our difficulties in arriving at a clear conception of this 

 well-styled " elusive " term. Some authors propose to distinguish species, as 

 morphologic, physiologic and so on, according to the basis of discrimination. It 

 is not clear, however, on what purely morphological ground the male and female 

 of many organisms can possibly be included in the same species and the same 

 difficulty would occur in the case of those organisms which assume entirely 

 different forms at different periods of their life history, e.g., Puccinia graminis. 

 Moreover, if this is not done, by destroying the base on which our natural 

 history classifications depend, we must necessarily alter the whole character 

 of such classifications, which, to say the least, would be a very drastic measure. 

 The following, again, is extracted from one of the most recent standard works 

 on Physiology: — 



" It has long been recognized that a species has no definite boundaries 

 but is a conventional expression for a cycle of forms grouped around a type- 

 centre*." 



This seems to imply that while modern naturalists exercise their individual 

 discretion in laying down the precise boundaries of species, they are agreed as to 

 the main principles to be followed in doing so. A comparison of the opinions 

 * Physiology of Plants by Dr. W. Pfiffer, Eng. Ed., 1903, Yol. II., p. 911. 



