nge in potency of the germ or of contamination of factors,
while on the contrary they are in entire accord with the view that he is
dealing with a case of modifying factors.

[Illustration: FIG. 89. Races of Paramecium. (After Jennings.)]

Equally important are the results that Jennings has obtained with certain
protozoa. Paramecium multiplies by dividing across in the middle, each half
replacing its lacking part. Both the small nucleus (micronucleus) and the
large nucleus (macronucleus) divide at each division of the body. Jennings
found that while individuals descended from a single paramecium vary in
size (fig. 89), yet the population from a large individual is the same as
the population derived from a small individual. In other words, selection
produces no result and the probable explanation is, of course, that the
different sizes of individuals are due to the environment, while the
constancy of the type is genetic. Jennings found a number of races of
paramecium of different sizes living under natural conditions. The largest
individual of a small race might overlap the smallest individual of other
larger races (fig. 89); nevertheless each kind reproduced its particular
race. The results are like those of Johannsen in a general way, but differ
in that reproduction takes place in paramecium by direct division instead
of through self-fertilization as in beans, and also in that the paramecia
were probably not homozygous. Since, however, so far as known no
"reduction" takes place in paramecium at each division, the genetic
composition of parent and offspring should be the same. Whether
pseudo-parthenogenesis that Woodruff and Erdmann have found occurring in
paramecium at intervals involves a redistribution of the hereditary factors
is not clear. Jennings's evidence seems incompatible with such a view.

[Illustration: FIG. 90. Stylonychia showing division into two. (After
Stein.)]

More recently one of Jennings's students, Middleton, has made a careful
series of selection experiments with Stylonychia (fig. 90) in which he
selected for lines showing more rapid or slower rates of division. His
observations seem to show that his selection separated two such lines that
came from the same original stock. The rapidity of the effects of selection
seems to preclude the explanation that pseudo-parthenogenesis has
complicated the results. Nevertheless, the results are of such a kind as to
suggest that they were due to selection o