402 THE EVOLUTIONARY SIGNIFICANCE OF SPECIES. 



gap between them. "Wliether 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 evo- 

 lutionary. Evolution continues whether the species is divided or not; 

 the divergence of the parts is rendered possible by the cessation of the 

 interbreeding, Avhich 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. Isoation has furnished millions of these tests of the 

 universality of biological motion, but it does not cause the motion. 

 Evolution is independent of isolation, and AA'ithout it has often 

 brought about in the same species great diversity of 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 di^'ergences sometimes assist natural selection or 

 physical barriers in the work of subdividing species, this would 

 mean that evolution sometimes results in segregation, not that segre- 

 gation causes evolution. 



Evolution is a process of change in species; it is the journey of 

 which individual variations are steps. Evolution changes the char- 

 acters of species, but it does not originate species; it makes sj^ecies 

 different when segregation affords the opportunity. 



Natural selection may assist geographical and other influences 

 tending to the division of species, but it is not on that account a cause 

 of evolution; it represents the determining aspect of the environ- 

 ment — the factors Avhich influence the direction of the vital motion, 

 but not those which induce the motion. Natural selection may explain 

 differences between two species, but not the becoming different. It 

 is an external incident or influence and not an active principle or 

 agency of organic evolution. Adaj^tation is possible because there 

 is a vital motion which can be deflected, not because the environment 

 changes the characters of species. The river of evolution flows 

 , through the land of environment; the conformation of the valley 



