598 
the methods of breeding that can be 
recommended for the use of practical 
breeders have changed but little in the last 
twenty years, the greatest change being 
primarily in the greater surety with which 
we now make recommendations. It is the 
speaker’s purpose in this address to em- 
phasize certain salient features of the ad- 
vance that has been achieved, and point 
out what he conceives to be some of the 
most important problems awaiting solution. 
Twenty years ago our understanding of 
the principles of breeding was derived 
largely from Knight’s physiological papers 
and Darwin’s ‘‘Origin of Species’? and 
“Plants and Animals under Domestica- 
tion.’’? Verlot’s admirable pamphlet ‘‘On 
the Production and Fixation of Varieties 
of Ornamental Plants’’ gave a general out- 
line of the best methods then followed, and 
we derived our knowledge of the use of 
hybrids largely from Focke’s excellent text, 
“Die Pflanzenmischlinge,’’ published in 
1880, and the work of the French experi- 
menter Naudin. 
At that time breeders clearly understood 
the fact that hybrids segregated in the sec- 
ond generation and gave new combinations 
of characters, and the suggestion was even 
then present in the minds of scientific 
breeders, that this segregation of characters 
took place during the reduction division. 
At that time breeders, just as definitely as 
now, planned experiments in hybridizing 
different varieties or species to secure cer- 
tain recombinations of desired characters 
in the hybrids. The experiments in citrus 
hybridization conducted by Mr. W. T. 
Swingle and the speaker were planned in 
1893 entirely on this basis, yet the prin- 
ciple was in no sense of the word original 
with us, but was at that time well under- 
stood by all practical breeders. This 
understanding, the speaker thinks, was 
largely derived from the investigations of 
SCIENCE 
[N.S. Vou. XXXV. No. 903 
Naudin, though various investigators con- 
tributed to it. 
With a full understanding of the knowl- 
edge and practises of the breeders of two 
decades ago, it must be admitted that the 
conception of unit characters and Men- 
delian segregation was necessary to clarify 
this knowledge and bring out the latent 
possibilities of the material presented by 
nature for the use of the breeder, and it is 
doubtful whether we even yet adequately 
comprehend the almost infinite possibilities 
open to us. 
To understand breeding to-day we must 
clearly understand the conception of unit 
characters. We no longer conceive the 
species, race or variety, as a fixed ensemble 
of characters. Following De Vries, we now 
commonly conceive the species or variety 
to be made up of a certain number of unit 
characters, that are in large measure as- 
sociated together by the accident of evolu- 
tion or breeding and which are separable 
entities in inheritance. We may liken 
these unit characters to bricks used in the 
construction of a building, each separate 
and yet dependent on the others for the 
maintenance of the structure; as each unit 
character is dependent on the other unit 
characters for the maintenance of the 
plant body. We may think of these unit 
characters as organic elements similar to 
chemical elements, that by their recom- 
bination through hybridization, form new 
compounds—new plants—of distinetly dif- 
ferent appearance, but which in turn do 
not affect the unit characters, which may 
again be separated and led to form other 
compounds, again resulting in distinct or- 
ganisms. Related species may possess 
many distinct unit characters, but ordi- 
narily would be expected to possess many 
similar unit characters. Cultivated races 
or varieties ordinarily would differ only in 
a few unit characters, and difference in a 
