IN ORGANIC EVOLUTION 77 



eties are recognized only as a geographical lim- 

 itation can be shown. 



The known facts fully justify the statement 

 by Dr. A. E. Ortmann that : — 



" The four factors named, variation, inheritance, 

 selection, and separation, must work together to form 

 different species. It is impossible to think that one of 

 these should work by itself, or that one could be left 

 aside." 



To use a convenient analogy, the movement of 

 organic evolution may be compared to the course 

 of a stream. Isolation is the rocky ledge which 

 does nothing, but whose resistance must deter- 

 mine the direction of the river's flow. Selection 

 is the force that drives the stream along, and vari- 

 ation and heredity lie inherent in the nature of the 

 stream of life itself. All of these are necessary 

 in bringing about the final result, whatever that 

 may be. With these there are doubtless other 

 facts, extrinsic and intrinsic, but in this world of 

 varied contour and of prodigal reproduction, no 

 organism, whatever its heredity or its variations, 

 can escape these limiting environmental condi- 

 tions. Whatever takes part in the final result 

 must be a factor in evolution, whether it be an 

 initial factor in variation or not. 



In the belief of the writer, the minor differ- 

 ences which separate species and subspecies 

 among animals and plants, in so far as these are 

 not traits of adaptation (and most of them are 

 clearly not such), owe their existence to some 



