THE INDIVIDUAL AND THE SPECIES 201 



fluctuate round the same mean values. Two or more 

 elementary species may have had the same origin — 

 a common ancestor or ancestors — but the organisms 

 in one species exhibit characters which, although 

 similar in nature to those of the other species, yet 

 fluctuate about different mean values. 



This is not the " species " of the systematic biologist. 

 The Linnean or systematic species is a concept which 

 is much more difficult to define : it is a concept indeed 

 which has not any clear and definite meaning, in 

 actual practice. 



We often forget how very young the science of 

 systematic biology is, and how intimately its progress 

 has been dependent on that of human invention and 

 industrial enterprise. Physics and mathematics might 

 be studied in a monastic cell, but the study of systematic 

 biology can only be carried on when we have ships 

 and other means of travelling — the means, in short, 

 of collecting the animals and plants inhabiting all 

 the parts of the earth's surface. Until a comparatively 

 few years ago the fauna and flora of great tracts of 

 land and sea were almost unknown : even now our 

 knowledge of the life of many parts of the earth is 

 scanty and inaccurate. Systematic biology has there- 

 fore had to collect and describe the organisms of the 

 earth, and in so doing it has set up the Linnean species 

 of plants and animals. These we may describe as, 

 in the main, categories of morphological struc- 

 tures. The older and more familiar species are clearly 

 defined in this respect : such are cats and dogs, 

 rabbits, tigers, herrings, lobsters, oysters, and so on : 

 the individuals in each of these categories are clearly 

 marked out with respect to their morphology, and the 

 limits of the categories are clearly defined. In all of 

 them the specific organisation has attained a high 



