ASPECTS OF KINETIC EVOLUTION 357 



amidst the apparently hopeless disorder of the stars. The 

 ancients, grouped the stars into constellations, but modern 

 science shows us systems ruled by laws of mathematical preci- 

 sion. 



Biology has remained longer in the constellation stage. Spe- 

 cies are still discussed, even by evolutionists, as though they 

 were mere chance aggregates of organisms, at once too familiar 

 and too diverse to be formally defined. 



It may well be that no coherent definition can be made for 

 species as mere aggregations or constellations of organisms ; 

 the idea itself is vague and essentially unscientific. The pri- 

 mary error was that of treating the species as a morphological 

 group, whereas the true evolutionary species is a physiological 

 system. Like a stellar system, it may contain a large number 

 of different individual members, and even different kinds of 

 members. The unity of the species does not depend upon 

 the organisms being all alike. It is necessary only that they 

 remain within range of mutual influence through interbreeding, 

 which is the biological analogue of gravitation. 



A species, that is, a normal, natural, evolutionary species, is 

 a large, coherent group of freely interbreeding organisms. But 

 with species, as with stars, all systems are not alike. There 

 are suns, satellites, planets, asteroids, nebulae, variable stars, 

 doubles and comets, in vast diversity of sizes and combina- 

 tions. 



In biology, as in astronomy, the most familiar things have 

 proved very deceptive. The sun, moon and stars appear alike 

 to revolve around the earth, from east to west. It was at first 

 an extremely heterodox idea that the earth revolves around the 

 sun. Moreover, neither of the apparent motions gave any inti- 

 mation of the third order of motion, that of the system as a 

 whole. In a similar way we have taken it for granted that the 

 evolution of species could be explained by the motions we have 

 been able to detect among our domesticated plants and animals. 

 We are now learning that these types of life are not reliable 

 examples of evolutionary systems, that their motions are often 

 retrograde or degenerative instead of progressive and construc- 

 tive. Nor are abnormal evolutionary conditions entirely con- 



