760 
research. When Mendel’s laws were redis- 
covered practically simultaneously in the 
closing year of the last century, by de 
Vries, Correns, and von Tschermak, biology 
received such a stimulus as it has not felt 
since the publication of the ‘‘Origin of 
Species’? in 1859. But what was it that 
gave this stimulus? Was it the collection 
of facts? Was it the wonderful collection 
of facts regarding variation made by Dar- 
win that electrified the scientific world in 
1859? Or was it a satisfactory interpreta- 
tion of these facts? Darwin gave his facts 
meaning. Similarly, it was an 2lwmina- 
ting interpretation of facts that made an 
epoch in the development of biological sci- 
ence when Mendel’s principles were redis- 
covered. The fact that this discovery lay 
unnoticed for a third of a century and then 
suddenly became the leading interest of 
biologists is a remarkable commentary on 
the relation of science to human welfare 
in the last century and in the present. 
The leading principle discovered by Men- 
del was that a hybrid whose parents differ 
in respect to a single factor of develop- 
ment, produces two kinds of gametes, re- 
spectively lke the gametes of the two 
parents. This is now known as the law of 
segregation. In the early years of the 
present century genetic investigations dealt 
mainly with the universality of this law. 
This question is now practically settled, so 
far as ean be by experimental cross-breed- 
ing. We now have a vast amount of data 
which need further interpretation that will 
point the way to new kinds of facts. The 
mass of data which has accumulated during 
the past dozen years has been variously 
interpreted by different investigators. It 
has furnished an extensive vocabulary of 
new terms, to which various meanings have 
been attached. Some recognized authority 
has suggested a new view concerning the 
nature of the so-called ‘‘unit-characters,”’’ 
SCIENCE 
[N.S. Von. XXXV. No. 907 
and this view has been widely accepted 
with no critical examination of its intrinsic 
merits. The weight of authority here, as 
elsewhere, has been an incubus on the prog- 
ress of scientific interpretation. 
Mendel himself did not make use of the 
term “‘unit-character,’’ but refers the phe- 
nomena he observed to differences between 
formative elements in the cells of hybrids. 
This is made clear in the following quota- 
tion from his original paper: 
With regard to those hybrids whose progeny is 
variable we may perhaps assume that between the 
differentiating elements of the egg and pollen cells 
there occurs a compromise, in so far that the 
formation of a cell as foundation of the hybrid 
becomes possible; but nevertheless the arrange- 
ment between the conflicting elements is only tem- 
porary and does not endure throughout the life of 
the hybrid plant. Since in the habit of the plant 
no changes are perceptible during the whole period 
of vegetation, we must further assume that it is 
only possible for the differentiating elements to 
liberate themselves from the enforced union when 
the fertilizing cells are developed. In the forma- 
tion of these cells all existing elements partici- 
pate in an entirely free and equal arrangement, 
in which it is only the differentiating ones which 
mutually separate themselves. In this way the 
production would be rendered possible of as many 
sorts of eggs and pollen cells as there are com- 
binations possible of the formative elements. .. . 
The differentiating characters of two plants can 
finally, however, only depend upon differences in 
the composition and grouping of the elements 
which exist in the fundamental cells of the same 
in vital interaction. 
Thus instead of ‘‘unit character’? Men- 
del speaks of ‘‘differentiating characters,”’ 
and instead of pangenes in the germ plasm 
he speaks of ‘‘formative elements,’’ differ- 
ences in which are responsible for the dif- 
ferences in related organisms. Nowhere 
does he advance the idea that the germ 
plasm is composed of independent ele- 
ments, each of which is responsible for the 
development of a definite portion of the 
organism. The latter idea is due to de 
