ISOLATION 171 



variation is more likely to be preserved than if the 

 number of the individuals was large. Even Dr. 

 Wallace allows that under these circumstances 

 characters which are neither advantageous nor dis- 

 advantageous will be retained. But he says that 

 they will be variable and not available for specific 

 distinctions. All specific distinction must be con- 

 stant, and, he thinks, useful. If this is correct it 

 would follow that no two species can have the same 

 environment, for each has some character specially 

 useful to that species only. If they live together, 

 under the same physical conditions, they must, 

 therefore, have different habits. And the characters 

 by which the different species are distinguished 

 must be related to their environment and be advan- 

 tageous to each. Any change of a character pos- 

 sessed by one species into that possessed by another 

 would be detrimental, unless it was accompanied by 

 a change in the environment. Let us try to test 

 this by an appeal to facts. 



It is almost impossible for us to observe plants 

 and animals so closely all through life, as to enable 

 us to say with certainty that any particular charac- 

 ter is of no use. But we can make inferences, and, 

 certainly, we should not assume that because many 

 characters are useful, therefore all are so. 



For example, many insects have well-developed 

 wings, yet they never use them ; and there are also 

 many others in which the wings are either rudimen- 

 tary or absent. In these cases we do not hesitate 

 to say that the useless wings were useful to former 

 generations. Again, there are other organs which 



