GENETICS: B. M. DAVIS 
705 
iana is likely to give up this habit of throwing variants however long 
it may be cultivated. 
Now the regularity with which Oenothera Lamarckiana produces 
its 'mutants' through successive generations indicates conditions within 
the germ plasm of such a nature that a number of different specific 
types of sexual cells are produced rather than a single set of gametes 
uniform in their germinal constitution. There is really not the spon- 
taneity in the production of new forms by Lamarckiana which one 
might expect of a plant in a state of 'mutation' with an organization 
expressing itself in irregular and unexpected departures from the type 
through peculiarities of mutating instability in its germinal constitu- 
tion. Consequently a critic of the evidence for the mutation theory 
offered by De Vries from the behavior of Lamarckiana very naturally is 
led to question the fitness of this plant as representative of a pure 
species. The discussion must finally center on the problem of whether 
or not the germinal constitution of O. Lamarckiana is homozygous, i.e., 
carrying two identical sets of hereditary factors derived from the parents 
through each sexual union. May not the germinal constitution be 
heterozygous, or hybrid, the two sets of hereditary factors in some 
respects differing from one another? 
An organism homozygous in germinal constitution can develop only 
one type of sexual cells, gametes, and these will be identical with those 
of the parents unless chemical or physical conditions affecting the germ 
plasm modify directly the germinal constitution carried through the 
succession of cell divisions that make up a generation, or upset the 
precision of the reduction divisions previous to the formation of gametes, 
or acting directly on the gametes themselves change their organization. 
Variations of the germinal constitution introduced in this manner would 
constitute mutations and it is an admitted fact that variations which 
might be interpreted as mutations are very rare in the lines of animals 
and plants which are beheved to be most pure and are consequently 
most stable in their breeding behavior. 
A heterozygous organism must at the time of gametogenesis distrib- 
ute the hereditary factors unevenly whenever these factors as they 
come from the two parental lines differ from one another. There are 
many reasons why hereditary factors are believed to be present in the 
chromosomes, and the reduction divisions which distribute whole chro- 
mosomes into two group clearly furnish a mechanism by which a segre- 
gation of factors may take place. The most complete and satisfactory 
studies on chromosome reduction for both animals and plants have 
