The Making of Species 
only will the most stable biological molecules be 
formed, but the most stable radicles will dominate 
the molecule. Hence, if any two animals are 
crossed and the offspring show alternate inherit- 
ance, the resulting organism will, in the case of 
each unit character, display the most stable of 
the pair; in other words, it will take after the 
parent which happens to have the greater 
stability as regards that particular character. 
The difference between the mule and the hinny 
would seem to be explicable on this supposition. 
If the union were like a simple chemical syn- 
thesis it should not make any difference which 
way the cross were made. But if the species 
crossed are of varying stability, and if their 
respective degrees of stability vary with the sex, 
it is easy to see that it will make a difference 
how the animals are crossed. 
In the cases of creatures that obey Mendel’s 
law, the most stable form of a unit character will 
presumably be the dominant one. 
One of the most curious of the phenomena of 
inheritance is that of correlation. We shall deal 
with this more fully in Chapter VIII. It will 
suffice here to say that certain characters appear 
to be linked together in organisms. Such seem 
to be transmitted in pairs. The offspring never 
exhibits one of such a correlated couple without 
exhibiting the other also. 
It would thus seem that certain combinations 
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