398 THE EVOLUTIONARY SIGNIFICANCE OF SPECIES. 



of evolution, but from everv mechanical or environmental stan(l})oint 

 species remain as mysterious as ever. 



A species is a species, not through the workings of any hidden cause 

 of evolution, hut because the component individuals breed together, 

 and thus remain in interconnected, coherent whole. This is the 

 familiar and obvious fact which has escaped the appreciation of spe- 

 cialists. The individuals of a species are alike, not because of any 

 fixed law or mechanism of heredity, but because they are traveling- 

 together along the evolutionary pathway. ^ATien a species encounters 

 an environmental obstacle, and is divided into two groups, the parts 

 can then evolve separately and become difterent. Evolution is not 

 shown in the becoming separate, but in the becoming different, and 

 the i^rocess of gradual change would haA'e continued if the original 

 group had not been subdivided. 



The policy of taking species for granted has been carried by some 

 to the point of declaring that no satisfactory definition of species 

 could be made, but this was only because they were attempting to 

 combine two incongruous ideas, and failed to distinguish the species 

 as the evolutionary unit of interbreeding individuals from the pre- 

 evolutionary species of analytical classification, " founded on identity 

 of form and structure." In nature, however, there is no identity of 

 form and structure, and no such species as those into which the nar- 

 rowly formal systematist subdivides his genera. 



It is not possible to make a definition which will enable us to 

 recognize species without studying them ; l)ut, on the other hand, it is 

 entirely practicable to tell what species are, or hoAv they are consti- 

 tuted. A species is nothing more nor less than a connected or co- 

 herent group of interbreeding organisms. Interbreeding may enable 

 them to maintain a general similarity throughout the specific series, 

 or there may be sexual and other diversification which subdivides 

 the species into two or more distinct sexes or castes without inter- 

 mediates. 



Adherence to the notion that species means " identity of form and 

 structure " brings hopeless confusion of evolutionary ideas. The 

 fact that characters which have arisen among inbred domesticated 

 plants and animals disappear in crosses Avith the prepotent wild 

 stock has l)een taken to prove " the swamping effects of intercross- 

 ing " in general. The segregation of new variations is now com- 

 monly held to be essential for their preservation, in complete disre- 

 gard of the fact that the inbreeding unavoidable in such segregation 

 Avould Aveaken the organism and give it a fatal handica[) in the strug- 

 gle for existence. The original assumption of this tlieory is as 

 erroneous as the final deduction. Instead of having, or tending to 

 haA'e, " identity of form and structure," it is a physiological advan- 



