ISOLATION AND SEXUAL SELECTION. 81 



lines of adaptation is to take place. The importance of 

 isolation has been variously estimated by different 

 authors ; but it must not be forgotten that it plays a 

 subsidiary part in evolution, and can do nothing with- 

 out selection and variation. Selection without isolation 

 may give rise to evolution in a straight line ; combined 

 with isolation it may lead to divergence into as many 

 lines as there are groups of individuals isolated. There 

 are several kinds of isolation : geographical, " physio- 

 logical," and isolation due to the adoption of different 

 habits and modes of life. 



Means of dispersal are various. Some organisms are 

 borne passively by the wind, as seeds and microscopic 

 plants and animals, others by water currents, and still 

 others by their own activity x move from place to place. 

 In one way or another species are always trying, so to 

 speak, to spread over a wider area. Land organisms 

 become geographically isolated by the formation of 

 barriers such as deserts, or mountain ranges, or by the 

 separation of parts of a continent as islands. Marine 

 organisms may be divided by the uprising of dry laud, 

 and the inhabitants of fresh water by the separation of 

 river basins. But whatever may be the barrier which 

 cuts off more or less completely a number of individuals 

 from the main stock, the result is the same they tend 

 to diverge from the parent species. This divergence is 

 due to differences in the environment directly or in- 

 directly modifying the individuals, to the new fauna and 

 flora with which the isolated specimens come into contact 

 altering the course of selection, and lastly to the appear- 

 ance of new mutations. So many local races or species 

 become differentiated ; and when a number of closely 

 allied forms occupy neighbouring regions, they are more 

 unlike, generally speaking, the further they have strayed 

 from the original centre of distribution. The more com- 

 plete and the older the barrier the greater will be the 

 divergence. Thus the marine and littoral fauna on the 



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