506 



SCIENCE. 



[N. S. Vol. XXIII. No. 587. 



a certain ' facies,' that is to say, each possess- 

 ing different ecological habits.^ 



Further particulars about the ecological 

 habits, geographical distribution and life his- 

 tory of the crawfishes mentioned above will 

 be given in my memoir on the crawfishes of 

 the state of Pennsylvania, which is now ready 

 for publication. A. E. Ortmann. 



Caenegie Museum, 



PiTTSBUKG, Pa., March 1, 1906. 



FACTORS OF SPECIES-FORMATION. 



To THE Editor of Science: The short note 

 by Dr. Ortmann in Science of January 12, 

 1906, no less than the larger work of Gulick 

 to which he refers, as well as much of the 

 recent discussion of isolation as an evolution- 

 ary factor, are all rich in illustrations of the 

 need of a simple distinction. 



Unfortunately for the progress of evolu- 

 tionary science among his contemporaries and 

 immediate successors, Darwin began the title 

 of his first book on evolution with the fateful 

 words, ' The Origin of Species.' Around this 

 ark of a new biological covenant the chosen 

 people of science have waged fifty years of 

 sanguinary warfare, and it is now a very un- 

 gracious task to convince them that the spe- 

 cies-origination box never did contain the 

 sacred relics of evolution. But if science is 

 to seek truth rather than tradition, we may 

 not close our eyes to the perception that the 

 factors of species-formation are not at all 

 factors of evolution. 



Questions of species-formation are gener- 

 ally debated because of interest in evolution, 

 though for purposes of scientific study and 

 explanation the two lines of investigation are 

 as completely distinct as gravitation and hy- 

 drostatics. Isolation, in one form or another,' 



' See Ortmann, ' Crustaceen ' in Semon, Zool. 

 Forsohungsreisen in AustraKen, etc., Jenaisch. 

 Denkschr., 8, 1894, p. 67; and Ortmann in 

 ' Bronn's Klass und Ordn. d. Tierreichs.,' 5, 2, 

 1899, p. 1,202. 



^ Chronological isolation may be quite as effect- 

 ive for species-formation as separation in space, 

 and permits species of common origin to become 

 diverse while still occupying the same region. 

 Many plants and insects, of tropical as well as of 

 temperate regions, have their flowering times or 



is an indispensable factor in the subdivision 

 of species, but to evolution it contributes 

 nothing whatever. Isolation may sometimes 

 retard or prevent evolution, but it is not an 

 evolutionary factor except in a minor and 

 negative sense.^ The two groups of phenom- 

 ena belong to entirely different categories; 

 stirring them together only keeps the emulsion 

 from clarifying into the two component solu- 

 tions. 



Evolution is a process of organic change 

 and development, universal and continuous, 

 and due to causes resident in species. Spe- 

 ciaiion, to give the other process a name, is 

 the origination or multiplication of species 

 by subdivision, usually, if not always, as a re- 

 sult of environmental incidents. Speciation 

 is thus an occasional phenomenon which does 

 not cause evolution, and is not caused by evo- 

 lution. One procession of organisms may be 

 divided into two, but it does not appear that 

 the new groups wiU travel in any different 

 manner than before, nor that they will go any 

 faster or any farther than if they had not 

 be'en separated. The subdivision enables the 

 two parts to follow different roads and to 

 arrive at different destinations, but it does 

 not assist the evolutionary locomotion nor give 

 us any clue as to how it is accomplished. The 

 evolutionary interest of isolation is that each 

 case affords additional evidence of continuous, 

 progressive change as the normal evolutionary 

 condition of all groups of interbreeding or- 

 ganisms. The isolation of a new group is an 

 interesting biological event, a crisis, as it were, 

 in speciation, but it gives us no special oppor- 

 tunities of studying the causes of evolution. 

 Perception of these elementary facts would 

 have saved the writing of many books, and 

 breeding seasons restricted to annual occasions of 

 extremely short duration. In some groups a 

 considerable series of years may intervene be- 

 tween periods of propagation, as in the bamboos 

 and periodical cicadas. 



"A more extended presentation of this distinc- 

 tion is to be found in ' Evolution Not the Origin 

 of Species,' Poptilar Science Monthly, March, 

 1904. The paper was reprinted in revised and ex- 

 tended form in the Smithsonian Report for 1904, 

 pp. 397-412, under the title ' The Evolutionary 

 Significance of Species.' 



