PHENOMENA OF INHERITANCE 219 



of the permutations of ancestral characters, 

 the appearance of mutations, and the fluctua- 

 tions of organisms due to environmental 

 changes, it happens that in all cases offspring 

 differ more or less from their parents and from 

 one another. No two children of the same 

 family are ever exactly alike (except in the 

 case of identical twins which have come from 

 the same oosperm) . Every living being ap- 

 pears on careful examination to be the first 

 and last of its identical kind. This is one of 

 the most remarkable peculiarities of living 

 things. The elements of chemistry are con- 

 stant, and even the compounds fall into 

 definite categories which have constant charac- 

 teristics. But the individuals of biology are 

 apparently never twice the same. This may 

 be due to the immense complexity of living 

 units as contrasted with chemical ones, — in- 

 deed lack of constancy is evidence in itself of 

 lack of analysis into real elements or of lack of 

 uniform conditions, — but whatever its cause 

 the extraordinary fact remains that every liv- 

 ing being appears to be unique. "Reproduc- 

 tion is the generation of unique beings that 



