448 POPULAR SCIENCE MONTHLY. 



mits more uniform progress. A single species may have as great a 

 variety of characters as a dozen related groups which have been segre- 

 gated. Two species may be quite distinct and yet differ much less than 

 the connected extremes of another. That a species differs in different 

 parts of its range does not necessarily mean that a subdivision will take 

 place; it means merely that characters are originating more rapidly 

 than they spread over the whole species. The integrity of a species is 

 not destroyed by 'inconstancy' of characters, but because geographical 

 or other barriers make a gap in the series. 



The failure of the extremes of a widely distributed species to breed 

 when brought together does not prove the attainment of specific dis- 

 tinctness, nor the approach of it, since internal diversity does not 

 weaken the species, but is an evolutionary advantage, and both extremes 

 may continue to cross freely with the connecting forms, which consti- 

 tute the bulk of the species. Neither does the power to form fertile 

 hybrids prove that two species occupying distinct ranges are one. Faith 

 in such criteria is simply a remnant of the pre-evolutionary theory of 

 the separate creation of species. The only way to ascertain that two 

 groups of organisms are separate species is to find the gap between 

 them. Whether they will breed together or not, and whether the 

 hybrids are fertile and vigorous, or weak, sterile and aberrant, may 

 indicate the period and degree of divergence of the types crossed, but 

 affords absolutely no evidence as to whether the series to which they 

 belong in nature are continuous or interrupted. Specific distinctness 

 is a question much more geographical than evolutionary. Evolution 

 continues whether the species is divided or not; the divergence of the 

 parts is rendered possible by the cessation of the interbreeding which 

 would otherwise maintain the coherence and relative uniformity of the 

 undivided group. 



Segregation and Vital Motion. 



The systematist 'separates' species because they are 'different,' but 

 the evolutionary significance of species does not appear from formal 

 descriptions of these biological islands; it lies in the fact that isolated 

 groups of organic individuals universally acquire diagnostic differences. 

 Isolation has furnished millions of these tests of the universality of 

 biological motion, but it does not cause the motion. Evolution is inde- 

 pendent of isolation, and without it has often brought about great 

 diversity of form an(J structure, as witness the dissimilar sexes, castes, 

 dimorphic and alternating generations of many species of plants and 

 animals. "Without evolutionary progress there would have been no 

 species as we now know them, but the causes of the segregation of 

 species are not causes of evolution; segregation merely permits this 

 universal tendency to become more manifest. If it should be found 

 that evolutionary divergences sometimes assist natural selection or 



