IOWA ACADEMY OF SCIENCES. 39 



directions around the normal, but wholly or mostly in a 

 favorable direction. 



Let us illustrate this by the case of an animal that com- 

 monly escapes its enemies by virtue of its speed. If con- 

 siderably speedier than the forms that prey upon it, it will 

 always escape when it has a chance to bring its powers of 

 flight into operation, and will be destroyed only by being 

 surprised or in some other accidental manner; but if it 

 differs but little in speed from its enemies the effort that 

 it will have to put forth to escape in each case of pursuit 

 will be inversely proportional to the amount of this differ- 

 ence. If its enemies in the course of generations increase 

 their speed, the animal preyed upon will likewise be com- 

 pelled to put forth greater efforts, and those individuals 

 that fire capable of responding best to this increased effort 

 by an increase in the development or their muscles, will be 

 the ones in each generation to survive. All the forces of 

 the environment will therefore act to produce a variation 

 in a desirable direction alone; while there will be in no 

 case, even where the animal may accidentally be freed 

 from the chances of pursuit, of any environmental forces 

 acting to produce a physiological change that would result in 

 a less degree of speed. In other w^ords there is a progress- 

 ive adaptation to the needs of the environment. 



I wish to present still another thought in regard to 

 forms in a new environment. We must remember that 

 wdien a species is subjected to changed conditions, these 

 changes are not likely to take place simultaneously over 

 the whole range of the species. Those individuals that are 

 capable of being modified little will be especially energetic 

 in seeking out conditions as near as possible like the old 

 ones; while those that become, by individual modification, 

 adapted to the new conditions, finding here less competi- 

 tion from their kind than in the overcrowded, because less 

 changed portions of the territory, will in consequence jjre/'er 

 these new conditions. Local segregations of easily mod- 

 ified individuals will thus occur, and by the interbreeding 

 of these, their capability of modification will be accent- 

 uated. There will thus gradually arise a differentiation in 



