EVOLUTION IN NATURE 163 



with graded environmental conditions is not neces- 

 sarily an indication of cause and effect; as Cockerell 

 has pointed out, such a condition might very well 

 arise from the separate formation and later over- 

 lapping of interbreeding forms. ^^ We may turn to 

 nature only for materials and suggestions. If we 

 would associate phenomena accurately with their 

 causes we must see them as they occur, and for 

 this we must turn to experimentation. 



Since the theories of evolutionary method and 

 their association with observations of living organ- 

 isms are an incomplete picture of evolution, it is 

 necessary for us to pick out their most useful dis- 

 closures, the most significant probabilities of evo- 

 lution which they make evident, and to associate 

 these things with such evolutionary series as we 

 know in order to determine the most probable 

 general conditions of the process. 



As to source of variations, we admit mutations, 

 but the modern use of the term has brought it to a 

 point where any change affecting the germinal 

 chromosomes must be regarded as a mutation. We 

 have associated mutations with factors of the ex- 

 ternal environment, both in the direct action of 

 rays on the chromosomes and in the development 

 of known mutant characters in response to en- 

 vironmental conditions. A number of scientists 

 have demonstrated beyond reasonable doubt the 



21 Science N. S., Vol. XXIII, pp. 145-146. 1906. 



