THE NATURE OF SPECIES 

 By John Walter Gregory 



Professor of Geology, University of Glasgow 



The final test of the theory of organic evolution is whether 

 the different kinds of animals and plants are fixed and 

 unchangeable or whether one kind may through its posterity 

 give rise to or pass into another kind, even though the pas- 

 sage may be so slow that the changes produced in the lifetime 

 of one observer are slight. No highly specialized organism 

 can be expected to develop into an altogether different organ- 

 ism ; there is no chance, for example, that a humble-bee will 

 give birth to an elephant, or that a club moss will develop 

 into a fruit tree. Nor can any decisive verdict as to the 

 natural evolution of new forms of animals or plants be given 

 from a consideration of the various breeds of sheep or dogs 

 or garden plants. New breeds can unquestionably be devel- 

 oped at the will of the breeder; but the fact that domesticated 

 animals can be varied by breeding does not necessarily show 

 that under natural conditions the progeny of one kind of 

 elephant can become another kind, or that a certain sort of 

 moss, if placed in a new environment, will become another 

 kind of plant. 



The work done by breeders shows the plasticity of living 

 forms, and like plasticity is seen in animals and plants living 

 under natural conditions, but the question is whether there 

 are in the animal and vegetable kingdoms any well-estab- 

 lished units between which distinctions — even comparatively 

 slight distinctions — can be marked by boundaries that are 



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