i8 Darwin, and after Darwin. 



would be easy to present abundance of additional facts 

 from other sources, all bearing to the same conclusion 

 —namely, that as a matter of direct observation, no 

 less than of general reasoning, any unprejudiced mind 

 will concede to the principle of indiscriminate isolation 

 an important share in the origination of organic types. 

 For as indiscriminate isolation is thus seen sooner or 

 later to become discriminate, and as we have already 

 seen that discriminate isolation is a necessary condition 

 to all or any modification, we can only conclude that 

 isolation in both its kinds takes rank with heredity 

 and variability as one of the three basal principles 

 of organic evolution. 



Having got thus far in the way of generalities, we 

 must next observe sundry further matters of com- 

 parative detail. 



I. In any case of indiscriminate isolation, or 

 apogamy, the larger the bulk of the isolated section 

 the more nearly must its average qualities resemble 

 those of its parent stock ; and, therefore, the less 

 divergence of character will ensue in a given time 

 from this cause alone. For instance, if one-fourth of 

 a large species were to be separated from the other 

 three-fourths (say, by subsidence causing a discon- 

 tinuity of area), it would continue the specific characters 

 unchanged for an indefinitely long time, so far as 

 the influence of such an indiscriminate isolation is 

 concerned. But, on the other hand, if only half a 

 dozen individuals were to be thus separated from 

 the rest of their species, a comparatively short time 

 would be needed for their descendants to undergo 

 some varietal modification at the hands of apogamy. 



