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THE AMERICAN NATURALIST [Vol. LI 



tedly directly upon the germ cells, and the results are not 

 usually challenged. In other cases it is claimed to be 

 only upon the soma, which then modifies the germ cells. 

 These latter claims, however, me^t with singular indif- 

 ference or even distrust on the part of other biologists. 

 Vulnerable places are too easily found, such as the lack 

 of adequate controls. Sometimes the environmental evo- 

 lutionist is charged with unwillingness or inability to 

 show his hand when pressed for further information. 

 Furthermore, it may seem strange that in a world of biol- 

 ogists, all anxious to solve the problem of the method of 

 evolution, and all so far as I am aware willing that that 

 method should be anything whatever, all of the important 

 supposed cases of permanent modification caused by 

 environment should be advanced by a handful of investi- 

 gators. 



To conclude: We have affirmed our adherence to the 

 principle that evolution in past time is to be explained by 

 phenomena that occur to-day. No processes that do not 

 occur in living things now may be assumed to have 

 occurred in living things formerly, unless there is plain 

 evidence that events not explainable in terms of modern 

 metabolism once occurred. Applying this principle only 

 to the origin of modifications, not to their preservation, 

 we have shown that animals are evolving now through 

 agencies within themselves, independent of the environ- 

 ment. Whether environment also produces permanent 

 modifications is questionable, with the burden of proof 

 still resting upon those who hold that it does. All of the 

 known steps of evolution may be explained as originating 

 from within the animals ' organization. There is no neces- 

 sity of appealing to any other mode of origin, except, 

 perhaps, to satisfy a certain type of imagination. In 

 view of these considerations, it seems not illogical to me 

 to suspect that evolution, at least among all but the very 

 low animals and plants, is usually if not always initiated 

 by a chemical change, either directly or indirectly pro- 

 duced, in the chromosomes of the germ cells ; that these 



