28 SCIENCE 
lines” of beans, barley, etc.. I might equally 
well have called them simply “clones,” as 
“clonal varieties” and “clones” should have 
identical meaning. The word “clone” (Greek 
k\wy, a twig, spray, or slip) was proposed by 
Webber® for “groups of plants that are 
propagated by the use of any form of vegeta- 
tive parts, such as bulbs, tubers, cuttings, 
grafts, buds, etc., and which are simply parts 
of the same individual seedling.” I believe 
that no violence will be done by extending this 
term to include animals which are similarly 
propagated by any asexual method, and I 
suggest the general adoption of the word 
“clone” for all groups of individuals having 
identical genotypic character, and arising by 
asexual reproduction of any sort, including 
apogamy (7%. e., so-called “parthenogenesis ” 
unaccompanied by a reduction division). 
For the purposes of my discussion in 1904 
the distinction between “pure lines” and 
“clones” was of no consequence, because the 
particular hereditary principle then under 
consideration was common to both. The same 
thing is no doubt true of many of the recent 
investigations of others, but it is well to re- 
member that there are certain fundamental 
differences between “pure lines” and “clones,” 
which render it very important to maintain 
the distinction between them. I will mention 
but two of these differences by way of ex- 
ample: (1) In the “clone” it is possible to 
retain as a permanent feature of the group 
any purely heterozygous character, as for in- 
stance the vigorous constitution dependent 
upon the stimulation of heterozygosis; such a 
phenomenon is impossible in the “ pure line.” 
(2) When clonal individuals reproduce sex- 
ually, either by self-fertilization or by crossing 
with other individuals, they need not, and 
usually do not, produce genotypically equal 
offspring, because the individuals of the 
*Scrmnce, XVIII., 501-503, October 16, 1903. 
For a discussion of the spelling of the word 
““elone’’? see SCIENCE, XXII., 89, July 21, 1905. 
®This restriction is necessary in order to avoid 
confusion through the appearance of bud-muta- 
tions. Such a mutation if propagated vegetatively 
represents the origin of a new clone. 
[N.S. Vou. XXXV. No. 888 
“elone’’ are not necessarily homozygous, as 
the individuals of the “pure line” generally 
are. The “clones” of horticultural plants are 
notorious for the heterogeneity of their seed- 
ling offspring. The investigator of inter- 
mittently parthenogenetic organisms like 
aphids, rotifers and Hieracium, and of inter- 
mittently vegetatively produced organisms 
like paramecium and many plants, can not 
properly assume that their races are geno- 
typically pure in the sense that they are 
homozygous, while the worker with “pure 
lines” can make such assumption with small 
probability of error, in case his self-fertiliza- 
tions have been controlled with adequate care 
during a sufficiently large number of gen- 
erations. 
There is another prevalent misconception 
regarding “pure lines,” to which attention 
needs to be called. The word “pure” in this 
connection does not refer to the genotypic 
equality of the individuals, but only to the 
exclusion of all crossing as a source of geno- 
typie differentiation. 
In Dr. Harris’s criticism” of Roemer’s 
work with peas, he points out with very evi- 
dent satisfaction that two of Roemer’s popu- 
lations are historically traceable to individual 
selections and that they are therefore really 
“pure lines” (2. e., providing, of course, that 
those originally selected individuals were 
strictly homozygous, and that no chance cross- 
ing has taken place since). If no genotypic 
changes can take place within the “ pure line” 
all evolution is impossible in the large num- 
ber of forms which naturally maintain “pure 
lines” by obligate self-fertilization. The only 
point to be made in regard to this feature of 
Roemer’s results, is that, if his populations 
were really “pure lines,” the numerous dis- 
tinct biotypes he discovered by the “pure 
line” method in those populations, were the 
result of mutations which have taken place 
since the original selections were made. The 
oceurrence of such mutations does not affect 
in the least the value of the genotype theory, 
nor the importance of the “ pure line” method 
for the study of heredity. 
0” Amer, Nat., XUV., 686-700, November, 1911. 
