Phenomena of Inheritance 7S 



single mutation or change in the hereditary constitution ; and yet 

 only the latter are of significance in heredity and evolution. This 

 distinction between variations due to environment (fluctuations) 

 and those due to hereditary causes (mutations) was recognized 

 by Weismann and many of his followers, but the actual demon- 

 stration on a large scale of the importance of this distinction is 

 due mainly to deVries. 



All hereditary variations, whether due to new combinations of 

 old characters or to the appearance of actually new characters, 

 whether small and continuous or large and discontinuous, have 

 their causes in the organization of the germ cells, just as do in- 

 herited resemblances. Heredity is not to be contrasted with var- 

 iation, nor are hereditary likeness and unlikeness due to con- 

 flicting principles; both are the results of germinal organization 

 and both are phenomena of heredity. 



4. Every Individual Unique. As a result of the permutations 

 of ancestral characters, the appearance of mutations, and the 

 fluctuations of organisms due to environmental changes, it hap- 

 pens that in all cases offspring differ more or less from their par- 

 ents 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 appears 

 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 constant, and even the 

 compounds fall into definite categories which have constant char- 

 acteristics. 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, indeed lack of 

 constancy is evidence in itself of lack of analysis into real ele- 

 ments or of lack of uniform conditions, but whatever its cause 

 the extraordinary fact remains that every living being appears to 

 be unique. "Reproduction is "the generation of unique beings 

 that are, on the average, more like their kind than like anything 

 else" (Brooks). 



