2 $6 WILLIAM LAWRENCE TOWER. 



no certain criterion that they arose discontinuously. Osborn has 

 presented from the findings of the fossil record, certain facts 

 concerning the phylogeny of characters in mammals that now 

 behave Mendelianwise in crosses, but, which arose, as far as the 

 data of paleontology can decide, by gradual and continuous 

 modification, and not by imitative jumps, and further in that 

 the Mendelian reactions concern only extremes or alternatives 

 in the same quality of the organism, it may result that the 

 entire Mendelian phenomena are but the expressions of char- 

 acters whose manifestation is capable of easy and reversible 

 manifestation. 



The fact that this alteration is inheritable at all, may to some, 

 seem incomprehensible, but it never segregates, its behavior in 

 heredity is always that of a mono-hybrid, and in all experiences 

 it behaves as a property characteristic of the whole, and possibly 

 involves, therefore, an alteration not of determiners in the usual 

 sense, but an alteration of the colloidal matrix of the tissues, 

 soma and germ, which we must regard not as different kinds of 

 substance, but as different expressions of one and the same 

 specific kind of living material. 



From this series of experiments, operations that are important 

 in population evolution, especially of introduced populations, are 

 certainly discovered, and has served in my hands for an entering 

 wedge into the experimental investigation of questions concern- 

 ing the evolution of populations, an aspect of evolution of which 

 there is extremely little experimental analysis or investigation. 

 In nature and the diverse products of the evolution phenomena 

 in nature, our classification, schemes of phylogeny, our experi- 

 ences are with populations of diverse things, in associated rela- 

 tions, both living and non-living, and more intimate knowledge 

 of, and a broader understanding of these population problems 

 seems most necessary for further progress in evolution investi- 

 gations. 



As an example of the manner, and the change necessary, and 

 its production in the alteration of an organism originally char- 

 acteristic of a mesophytic habitat into one in all ways adjusted 

 to the rigors of the Arizona deserts, of the nicety of adjustment, 

 its promptness, and direct adaptive nature, these experiments 



