THE BREAKDOWN OI- CONTINUITY 



not exposed to this test they will not be eliminated. One consequence 

 of tliis we have seen. When we inbrced a normally outbreeding 

 stock, combinations become exposed in the homozygous condition 

 for the first time. Their internal balances have never been tested and 

 refined by natural selection, and in consequence they are mainly 

 poor, hibrccding depression is the result. 



In just the same way, by crossing individuals which would not 

 normally interbreed, we can obtain combinations in an untried 

 heterozygous relation. If they have been free to change since the 

 lines of descent diverged to give the individuals that carry them, 

 these combinations will generally not be capable of working 

 adequately together. Their joint action will not have been kept up 

 to an adequate level by natual selection: balance is not stable, 

 except in inbreeders. It decays if it is not being constantly adjusted 

 by selection. 



The complete prevention of crossbreeding by some geographical 

 obstacle wiU therefore mean that the genie combinations of the 

 separated groups wiU gradually lose their relational balance, provided 

 that within at least one of them sufficient outbreeding occurs not to 

 freeze the flow of variability and so prevent all genetic change. 

 As time goes on, the hybrid made by artificial intercrossing of the 

 groups will come to be inadequate genetically. It will show an 

 abnormal character or reduced fertility, more often the latter. The 

 groups will ultimately become separated by the bar of genetical inter- 

 sterility. They will be separate species. The same effect may, of 

 course, also be brought about by ecological or seasonal isolation. The 

 isolation, and not the means by which it is brought about, is the 

 cause of the decay in relational balance. 



Where isolation by environmental agencies is incomplete, the 

 decay of relational balance will be slower until genetical bars to 

 crossing also arise. Soon after crossing becomes restricted between 

 groups, the product of such crossing will begin to show the effects 

 of the restriction in the form of some measure of hybrid incapacity, 

 some failure in growth or reproduction. Such progeny are a waste 

 of reproductive effort, and the less the number that an individual 

 produces, the greater its chance of contributing to posterity. Any 

 gene, therefore, which further restricts the rate of crossing between 

 groups in individuals which bear it, will confer a selective advantage 



306 



