The Laws of Species Forming^ 



Bv DAVID STARR JORDAN 



SOME years ago I ventuiTd the 

 statement that taking any given 

 species of animal, the nearest 

 rehited form would not be found along 

 with it, nor at a great distance, but on 

 the other side of some barrier which 

 prevented intermingling. 



This fact is almost self-evident, as far 

 as the higher animals are concerned. 

 We very rarely have any difficulty in 

 determining the separate species in any 

 given region. For this reason those 

 naturalists who read the proposition 

 have accepted it. Dr. J. A. Allen, one of 

 the highest authorities on birds, went a 

 little further and named it "Jordan's 

 Law." But it existed in the nature of 

 things before it was mine, and I can be 

 credited with its ownership only for the 

 sake of convenience in the discussion of 

 the many ways in which life manages 

 itself, which we call natural "laws." 



This law goes back to the origin of 

 diversity in life, which, among other 

 ways, shows itself in what we call species. 

 A species of animal or plant is one of the 

 many kinds into which organisms are 

 divided. It has no objective, definition, 

 or criterion. It is merely one particular 

 crowd or mass of living things, giving 

 rise by processes of reproduction to a 

 succession of similar organisms, not all 

 alike but nearly alike, so that for ordi- 

 nary scientific purposes one name may 

 serve for all. 



If however, its range be broken by a 

 barrier of some sort, so that interming- 

 ling and interbreeding are no longer 

 possible, the mass will be separated. 

 Those on one side of the barrier will 



' Abstract of address before the American Associa- 

 tion for the Advancement of Science, San Diego, 

 California, August 10, 1916. 



gradually throw stress on one sort of 

 characters, those on the other side on 

 something else. The original stock may 

 average dift'erent or the two groups 

 may be subjected to different forms of 

 selection, or the stress of a different en- 

 vironment, and the final result will be 

 different. If these forms have sufficient 

 sharpness of definition we call them dif- 

 ferent "species," even though the differ- 

 ences be small. If however, the separa- 

 tion be imperfect, so that intermediate 

 forms lie along the road, we call them 

 "subspecies." Inside every species or 

 subspecies we find a differing range of 

 individual variation. As a rule those 

 variants w^hich arise in the midst of a 

 species are soon lost, unless they can be 

 isolated or segregated in some way — 

 and this rarely occurs in nature. Great 

 differences can be produced and empha- 

 sized in the domestication of many 

 species of animals or plants. This is 

 called artificial selection, but it depends 

 not only on selection but also on the 

 segregation or isolation of the product. 

 The races of dogs would never have 

 arisen if the animals chosen to be friends 

 of man, had continuously bred freely 

 with the mass of wolves. 



In discussing the origin of species, we 

 premise first that species in nature 

 exist — but not as closed categories. 

 They arise through the perpetuation by 

 heredity of variants which are cut off 

 from the original species by some sort 

 of barrier of land or sea, of temperature 

 or climate, of food or enemies. Once 

 cut off, their inadaptive excrescences 

 are pared away by selection, and at last 

 the new species, as well as the old, comes 

 to fit its environment as the river fits 

 its bed. 



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