338 BET. J. T. GULICK ON 



indiscriminate destruction cannot be classed as a form of Natural 

 Selection, it may nevertheless be the cause of transformation ; 

 and when a species is distributed in sections that are prevented 

 from intergenerating, divergent evolution will often be hastened 

 by the indiscriminate destruction of part of the members of one 

 or more of the sections. If a species inhabiting a large island is 

 divided by geological subsidence into v\xo equal sections, there 

 may be a very close resemblance in the average character of the 

 two sections ; but if a subsequent eruption of hot ashes destroys 

 a large portion of the individuals of one section, or of both, the 

 probability of a close correspondence in the average character 

 of the two sections will be very much less than before the 

 eruption. 



Again, when an area occupied by a species is divided into two 

 or more equal districts, the occupants of which can have little or 

 no opportunity for crossing, divergent evolution will arise in the 

 different districts unless there is some constantly operating cause 

 that ensures all the varieties that survive and propagate in any 

 district shall survive and propagate in all the districts. No such 

 cause has ever been pointed out ; but, on the contrary, it can 

 easily be shown that the probability is very small that such a 

 correspondence would occur, even if at the time of the division 

 of the area every individual in each district was represented by a 

 completely similar individual in each of the other districts. Let 

 us suppose a case : — 



1. Suppose the creatures under consideration to be a species 

 of mollusk, the sexual instincts of which act without any segre- 

 gative tendency between the varieties of the same species, there 

 being no aversion or other impediment that interferes with the 

 free crossing of all the variations occurring within the limits of 

 one district. 



2. Suppose that the number of individuals in each district is 

 10,000,000. 



3. Suppose that one in a thousand of these had a tongue strong 

 enough to feed on the bark of the tree, the leaves of which are 

 the ordinary food of the species, and that one in a thousand is 

 capable of digesting the same, so that, in each district alike, one 

 in a million could survive in this way though the crop of leaves 

 should fail. 



4. Suppose that there are, through diversity of adaptations of 

 this kind to the products of the environment, ten different kinds 



