504 KEY. J. T. GULTOK ON DIVEBSITY OF EVOLUTION. 



Changes ^produced hy tJie Introduction of Enemies. 



At this point one other inquiry naturally arises : — If the multi- 

 tude of varieties and the restricted distribution of both varieties 

 and species is in any degree due to freedom from severe compe- 

 tition, what would be the effect if, by degrees, many birds and 

 insects, hostile to these snails, should lind their way to the Sand- 

 wich Islands and become numerous in those mountain-regions ? 

 One of the first effects would naturally be the disappearance of 

 many varieties and species by which the different forms of each 

 genus are now so minutely gradationed together. Certain pro- 

 tective colours would be made to prevail, to the partial exclusion 

 of some of the brilliant contrasts of colour. The same enemies 

 being found in all the valleys of an islaud, the forms that proved 

 to be best fitted to sui'vive in one valley would have the advantage 

 everywhere, and therefore gradually spread from valley to valley. 

 The distribution of species and their separation from each other 

 by distinct forms Avould thus become similar to Avhat is found in 

 the case of continental species. 



The destruction of forests by the introduction of cattle and 

 goats is now causing the extinction of some of the species, 



Recapitulation and Conclusion. 



A compai'ison of the distribution of island mollusks Avith the 

 widely contrasted distribution of continental species, leads me to 

 believe that the evolution of many different species may take 

 place without any difference in the food, climate, or enemies that 

 surround them. The rapidity of evolution or the time within 

 which a certain amount of change is effected must depend upon 

 the average amount of change in one direction in a single gene- 

 tation, and the rapidity of succession in the generations. Ten 

 thousand years would make but little difference in a species of 

 cedar, in which the life of a single tree might count a third of that 

 period. But in the case of some species of insects the same 

 period might covcu' ten thousand generations ; and though the 

 change in each generation might be as imperceptible as in the 

 cedar, the aggregate of change for the whole period might be very 

 apparent. 



We must also bear in mind, the Natural Selection arising from 

 severe competition with species that have a wide range tends to 

 prevent variation and give a wider diffusion to forms that would 



