96 CIRCUMSTANCES FAVORABLE TO THE 



gallic beings extremely low in the scale, which do not 

 propagate sexually, nor conjugate, and which cannot pos- 

 sibly intercross, uniformity of character can be retained 

 by them under the same conditions of life, only through 

 the principle of inheritance, and through natural selection 

 which will destroy any individuals departing from the 

 proper type. If the conditions of life change and the form 

 undergoes modification, uniformity of character can be 

 given to the modified offspring, solely by natural selection 

 preserving similar favorable variations. 



Isolation also is an important element in the modifica- 

 tion of species through natural selection. In a confined or 

 isolated area, if not very large, the organic and inorganic 

 conditions of life will generally be almost uniform; so that 

 natural selection will tend to modify all the varying indi- 

 viduals of the same species in the same manner. Inter- 

 crossing with the inhabitants of the surrounding districts, 

 will also be thus prevented. Moritz Wagner has lately pub- 

 lished an interesting essay on this subject, and has sho\7n 

 that the service rendered by isolation in preventing crosses 

 between newly-formed varieties is probably greater even than 

 I supposed. But from reasons already assigned I can by no 

 means agree with this naturalist, that migration and isola- 

 tion are necessary elements for the formation of new species. 

 The importance of isolation is likewise great in prevent- 

 ing, after any physical change in the conditions, such as 

 of climate, elevation of the land, etc., the immigration of 

 better adapted organisms; and thus new places in the 

 natural economy of the district will be left open to be 

 filled up by the modification of the old inhabitants. Lastly, 

 isolation will give time for a new variety to be improved at 

 a slow rate; and this may sometimes be of much import- 

 ance. If, however, an isolated area be very small, either 

 from being surrounded by barriers, or from having very 

 peculiar physical conditions, the total number of the in- 

 habitants will be small; and this will retard the production 

 of new species through natural selection, by decreasing the 

 chances of favorable variations arising. 



The mere lapse of time by itself does nothing, either for 

 or against natural selection. I state this because it has 

 been erroneously asserted that the element of time has 

 been assumed by me to play an all-important part in modi- 



