CH. xi.] TWO OBJECTIONS CONSIDERED. 37 



and accumulate variations on either side of a hitherto 

 established mean. Such a conclusion is implied by the 

 theory of natural selection, according to which specific vari 

 ation occurs, not in conformity to some mysterious law 

 of progress uniformly operating, but only in conformity 

 to some more or less conspicuous alteration in the sum- 

 total of the conditions of existence. 



It follows, therefore, that in general, when incipient 

 varieties are differentiated into well-marked species, the 

 number of intermediate forms must be immeasurably smaller 

 than the numbers of forms contained in the resulting species 

 to which they serve as the transition. During epochs of 

 rapid divergence, the means may all be extinguished after a 

 few hundred generations, while the generations of the ex 

 tremes which persist thereafter may be numbered by tens of 

 thousands. Suppose, for example, two great islands sepa 

 rated by a shallow sea. During long ages, while the floor of 

 this intervening sea is constantly rising, the specific changes 

 occurring on either island may be quite few and unimportant, 

 and such fossil records as are left will indicate a general per 

 sistence of type. But when in course of time the process of 

 elevation has converted this intervening channel into an 

 isthmus connecting the two islands, there must inevitably 

 ensue a marked change in the conditions of existence in 

 both regions. Extinction will go on at a relatively rapid 

 pace; and, as above illustrated, this extinction must ordi 

 narily result in the disappearance of intermediate forms and 

 the preservation of extremes. After a while this process 

 must result in the establishment of an approximate equili 

 brium among the forms of life over both areas, such . as 

 formerly obtained over each area separately. And thus for a 

 long time to come, the specific changes occurring will again 

 be few and unimportant. 



Thus we see graphically illustrated the truth that, in com 

 parison with the myriads of individuals comprising the well- 



