Makch 30, 1906.] 



SCIENCE. 



507 



many unprofitable controversies. General 

 theories of evolution are not to be established 

 on speciation, either by natural selection, iso- 

 lation or mutation. 



Gravitation furnishes what might be called 

 a background for hydrostatics, and evolution, 

 in a similar way, makes speciation possible. 

 Isolated groups of organic individuals always 

 become different. The vital equilibrium of 

 specific bodies of organisms is sustained by 

 interbreeding and evolutionary motion. An- 

 alogies of physical phenomena of rest and 

 inertia do not apply. To hold organic types 

 uniform and stationary by selection has been 

 attempted many times, but degeneration 

 promptly ensues. Diversity and change are 

 not the results of special evolutionary causes 

 acting at rare intervals of species-formation, 

 but are the normal and necessary conditions 

 of organic existence. 



To understand species-formation, evolution 

 must be taken for granted, as an Irishman 

 might say, and as many savants of other 

 nationalities have unconsciously written. If 

 we are interested merely in the positional re- 

 lations of the automobile, it is enough to know 

 that the handle can be turned and that the 

 wheels go round. Our progress can then be 

 nicely explained by a few properly selected 

 factors, such as: (1) Wheel-turning (con- 

 tinued, inherited ' variation ') ; (2) roads to 

 travel (by selection) ; (3) handle-turning (ac- 

 commodation), to steer around corners and 

 mud-holes. But to ask how the machine was 

 constructed, how the handle turns it, and how 

 the wheels happen to revolve, are bothersome 

 questions of details with which taxonomic 

 observers of automobiles do not need to con- 

 cern themselves. Species travel because they 

 are built that way, not because the environ- 

 ment pushes them.'' Each species is equipped 

 on the inside with the factors of its own evolu- 



' Darwin and some of his followers appear to 

 have tacitly assumed what might be termed a 

 specific constant of variability, so that natural 

 selection by shearing off one side of the species 

 could compel the other side to grow out, and thus 

 roll the species along. How isolation could serve 

 as an evolutionary factor seems not to have been 

 indicated. 



tion, such as heterism, symbasis and mitapsis, 

 for maintaining the normal individual di- 

 versity and the broad network of descent 

 which are requisite for sustained organic effi- 

 ciency and evolutionary progress.* But all 

 this is another story. The factors of species- 

 formation afford very interesting matters of 

 discussion, but let us not confuse ourselves 

 further by imagining that they are factors of 

 evolution. 



O. F. Cook. 

 Washington, D. C, 

 January 27, 1906. 



SPECIAL ARTICLES. 



THE POSSIBILITY OF PSYCHICAL FACTORS IN 

 ILLUSIONS OF REVERSED MOTION. 



One of the most interesting chapters in 

 psychological optics is concerned with what 

 have been variously termed ' after-images of 

 motion,' ' antirheoscopic phenomena,' ' subjec- 

 tive complementary movements,' or ' illusions 

 of reversed motion,' These illusions are quite 

 easily observed, e. g., by fixating a rotating 

 disc on which a heavy spiral line has been 

 traced, or a rotating drum on which lines have 

 been drawn at right angles to the direction 

 of movement, or by watching the landscape 

 from the window of a moving train or the 

 waves of a stream from its bank. A very 

 pretty demonstration (the ' water-fall illu- 

 sion,' first described by Addam in 1834) may 

 be secured by fixating for a half-minute some 

 convenient mark seen through the falling 

 spray of a water-fall, and then transferring 

 the gaze to a neighboring cliff, which will 

 promptly ' flow ' upward in a most striking 

 manner. Indeed, some observers experience 

 all the unpleasantness of a vertigo from this 

 simple experiment, and several writers relate 

 the two phenomena by giving similar theo- 

 retical explanations. 



These illusions of reversed motion have been 

 under observation from time to time since the 

 first published account by Purkinje in 1825, 

 and have been experimentally examined in 

 numerous ingenious ways, first by Plateau in 



' ' Evolution of Cellular Structures,' Bull. 81, 

 Bureau of Plant Industry, U. S. Department of 

 Agriculture. 



