80 Experimental Zoology 
find evidence in its support, it is unprofitable as an interpreta- 
tion until or unless we can show that it is at least probable. 
The preceding discussion is based on the idea that there must 
be two kinds of germ-cells in order to give the Mendelian re- 
sults. This is perhaps the simplest way to formulate the prob- 
lem, but it should not be overlooked that the results can also 
be explained by assuming that all the germ-cells of the hybrid 
are alike, containing both dominant and recessive characters, 
but that after fertilization internal or external factors determine 
whether the one or the other character dominates. The diffi- 
culty of this view is to account for the middle term of the Men- 
delian proportion, in which, although the dominant develops, 
the germ-cells seem to return to the two original kinds. Pos- 
sibly this difficulty could be met by assuming that the two con- 
trasted characters are so nearly balanced in half of the progeny, 
that while the dominating character dominates when differen- 
tiation begins, it is not able to do so in the germ-cells. It is 
evident, however, that this interpretation is more complicated 
than that of two kinds of germ-cells, and fails to account for 
the three sharply marked groups. 
LITERATURE, CHAPTER VI 
Bateson, W. Mendel’s Principles of Heredity. 1902. 
Note on the Resolution of Compound Characters by Cross-Breeding. 
Proc. of the Cambridge Philos. Soc. XII. 1902. 
On Mendelian Heredity of Three Characters Allelomorphic to Each 
Other. Proc. Cambridge Philos. Soc. XII. 1903. 
CasTLzE, W. E. The Laws of Heredity of Galton and Mendel, and Some 
Laws governing Race Improvement by Selection. Proc. of the 
Amer. Acad. Arts and Sci. XXXIX. 1903. 
Mendel’s Law of Heredity. Proc. Amer. Acad. Arts and Sci. 
XXXVIII. No. 18. 1903. 
Mendel’s Law of Heredity. Science, n.s. XVIII. 1903. 
Correns, C. G. Mendel’s Regeln ueber das Verhalten der Nachkommen- 
schaft der Rassenbastarde, Ber. deutsch. bot. Gesellsch. XX. 
1900. 
DarsisHtre, A. D. On the bearing of Mendelian Principles of Heredity 
on Current Theories of the Origin of Species. Mem. and Proc. of 
the Manchester Liter. and Philos. Soc. XLVIII. 1904. 
Focxe, W. Die Pflanzen-Mischlinge. Berlin, Borntraeger. 1881. 
MENDEL, G. Versuche ueber Pflanzenhybriden. Verh. naturf. Vereins in 
Briinn, IV. 1866. 
