266 GENETICS IN RELATION TO AGRICULTURE 
The Nature and Causes of Factor Mutations.—Our knowledge of 
genetic factors is entirely of an inferential sort and it is probable that 
these ultimate hereditary units are no more likely to be objectively 
perceived than are the atoms of which all matter is generally believed 
to be composed. But our present understanding of biochemistry and the 
chromosome mechanism of heredity leaves no room for doubt concerning 
the theoretical nature of these factors. Living protoplasm is generally 
considered as composed of very complex organic compounds. The 
phenomena of stereochemistry, especially the substitutional or cyclic 
changes which occur within various compounds under proper con- 
ditions, suggest that similar compensatory relations exist between 
the substances composing the living cell. Yet cytological observations 
indicate that the chromatin is the only permanent constituent of the 
nucleus and that the chromosomes are unaffected by the regular physio- 
logical processes of metabolism, growth and reaction to stimuli even 
though they play a very definite réle in all these activities. As was 
explained in Chapter IV, the chromosomes are linear series of loci whereat 
are located specific factors. According to the multiple allelomorph 
hypothesis more than one factor may exist at a given locus. Since the 
chromosomes appear to consist of the only permanent substance in the 
nucleus, it is conceivable that at each locus there exists a unique chemical 
system; yet it is not unreasonable to suppose that occasionally substi- 
tutional changes similar to those known to take place in less complex 
organic compounds may occur here. 
The contributions of Reichert on the specificity of proteins and 
carbohydrates as a basis for the classification of animals and plants are 
based on the fact that such substances as serum albumin, hemoglobin, 
glycogen and starch exist in stereoisomeric forms. That is, “each kind 
of substance may exist in a number of forms, all of which forms have the 
same molecular formula and the same fundamental properties in common, 
but each in accordance with variations in intramolecular configuration 
has certain individualities which distinguish it from others. ... It has 
been found that the number of possible forms of each substance is de- 
pendent upon the possible number of variations of the arrangements of 
the molecular components in the three dimensions of space, or, in other 
words, of variations of molecular configuration, the possible number in 
case of each substance being capable of mathematical determination. 
Thus, we find that serum albumin may exist in as many as a thousand 
million forms. Hemoglobin, the red coloring matter of vertebrate blood, 
is a far more complex carbon compound than serum albumin, and theoret- 
ically may exist in forms whose number is beyond human conception, 
running into millions of millions. The same is true of starch.” Having 
in mind this complex molecular structure of protoplasmic constituents 
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