78 



that in islands geologically recent (which often implies, 

 however, their existence through epochs which would 

 sound vast indeed to ears unscientific) we must not in- 

 variably expect to discover evidences of this law. On 

 the contrary, we must first of all take into account the 

 age of their formation, before we can judge a priori as 

 to the probability of its operation through a sufficient 

 interval of time to have become conspicuous in its 

 effects. I say " through a sufficient interval of time," 

 because the process of deterioration may be silently 

 going on, even now, in many an island, which has not yet 

 shown any matured traces of its action, except perhaps 

 in the case of a few species which appear to be more 

 particularly susceptible to contingencies from without. 

 We should then call to mind, that an enormous propor- 

 tion of nearly every insular fauna is composed of acci- 

 dental colonists during the last few centuries, in which 

 civilization and commerce have been unintentionally at 

 work in the cause of animal diffusion ; and that, there- 

 fore, if modifications in outward contour have not neces- 

 sarily resulted during a positive geological interval, it 

 would be absurd to look for them in the mere settlers 

 (as it were) of yesterday. 



Thus, it will be perceived, how necessary it is to take 

 every element and contingency into account before we 

 venture to pronounce dogmatically on either the exist- 

 ence or non-existence of any physical law; and how 

 cautious we should be of denying the legitimate opera- 

 tion of external influences in one region, because they 



