484 University of California Publications in Zoology [Vol. 12 



conclusion that heritable variations are produced as a direct 

 response to external stimuli, even if they cannot be shown directly 

 to favor it. 



It is noteworthy that Tower insists that although heritable vari- 

 ations are produced as a direct response to external stimuli, the 

 response itself is absolutely determined within the organism. To 

 quote his own words (1906, p. 295) : 



It is true that different intensities of the same stimuli call forth different 

 responses, but, as is shown in the chapter on coloration, the response is 

 entirely determined within the organism, which is adjusted to different 

 intensities of stimuli and reacts according to its own method and on the basis 

 of its own constitution, there being no specific reaction called forth by a 

 given stimulus. 



The facts from the study of the ' zoogeography of the higher 

 vertebrates appear to harmonize with these conclusions of Tower's. 

 As the evidence becomes more complete it appears more and more 

 clear that Darwin was right in assigning to the "nature of the 

 organism" a more important place in considerations of speciation 

 than that accorded the "nature of the conditions". 



Do we not have here a suggestion as to how isolation works? The 

 shaping of the species seems to depend upon two things primarily: 

 (1) the nature, rapidity, and intensity of the blows from the 

 environmental hammer; and (2) the nature of the organism itself. 

 Thus isolation apparently operates, in the higher vertebrates, by 

 segregating different sorts of environments and insuring their con- 

 tinued moulding of the particular segregated lots of individuals. 

 On passing from one faunal area, association, or life-zone to another 

 we encounter different, but closely related species. Thus differences 

 between geminate species (Jordan, 1908) are apparently dependent 

 upon the fact that differential environmental hammers have been 

 used on what was originally the same stock. On making a com- 

 parison of this sort, we note that there has been great diversity of 

 response on the part of the different members of the assemblage to 

 the same environmental complex. The entire assemblage in any 

 given district has been subject to the same environment, but the 

 assumed differences in the stuffs moulded in each case readily 

 account for the fact that all have not responded in the same way. 



Tower (1906, pp. 286-296) found that by subjecting a given 

 species of potato beetle to unaccustomed environmental stimuli 

 there resulted in the offspring a break-up into several forms, some 



