254 LIFE: ITS NATURE AND ORIGIN 



The difficulties here expressed are rather of a semantic nature. 

 The word "evolution" subsumes a great number of physical hap- 

 penings at a variety of structural and organizational levels, ex- 

 tending from the atomic and molecular, to plant and animal 

 groups having continental and even world-wide distribution. It 

 is seldom possible to think of and reason with phenomena emerg- 

 ing at any one level, in terms of the structural or organizational 

 units of some other level, especially if it be a remote one. Thus 

 the farmer understands crops in terms of choice of seed, planting, 

 cultivation, weeding, fertilizers, water and weather conditions, 

 etc., without even knowing anything about the chemistry or 

 genetics involved, though these underlie his success or failure. 

 Similarly, chemistry and genetic factors underlie the various 

 evolutionary phenomena emerging at higher levels. Since these 

 factors operate through heritable changes in biocatalysts, we may 

 justly consider such biocatalyst changes as the basic or primal 

 cause of evolution. 



What catalysts undergo heritable changes or additions? Not so 

 long ago geneticists would have answered: Heritable change is 

 due to gene mutations. Later, because of the "position effect," 

 the word "mutation" was extended to include certain heritable 

 chromosomal abnormalities affecting gene action. Dobzhansky 

 states 15 "In a wide sense, any change in the genotype which is not 

 due to recombination of Mendelian factors is called mutation. 

 In the narrower sense, it is a presumed change in a single gene, a 

 Mendelian variant which is not known to represent a chromosomal 

 aberration." 



The inheritance of non-chromosomal and non-genic catalysts is 

 still under investigation, and the importance of cytoplasmic in- 

 clusions has been already mentioned. It is generally admitted 

 that the particulate units known as plastids, found in plant cells, 

 are independent constituents of the cell in regard to heredity. 

 Viruses are also known to live in the cytoplasm of some cells, and 

 to be transmitted from one generation of cells to another. The 

 cytoplasm of the gametes must also contain specific particulate 

 units which govern differentiation. Physical or chemical changes 

 in any of these cytoplasmic inclusions may persist and may be 

 carried by heredity; and so also may the results of addition of new 

 or stranger molecules, as is the case in cells treated with cancer- 

 producing substances (e.g., 3:4-benzpyrene), or with yeasts which 

 become "trained" to ferment new sugars. 



