350 THE FUTURE OF ARID LANDS 
greater tolerance to differences in environment. The program is 
based on the fact that in a predominantly apomictic plant, occa- 
sional hybrid seeds may be obtained, which in turn develop into 
plants that are essentially apomictic. This obviates the necessity 
of spending years to stabilize a desirable type. Just how much this 
type of program will contribute to production of arid lands re- 
mains to be seen. 
In the self-pollinating cereals, Hordeum vulgare and Triticum 
aestivum, somatic variability is carried between lines only, and the 
population consists of a small number of homozygous biotypes. 
Mixtures of such biotypes planted, harvested, and reseeded for 
several years at a number of locations result in a preponderance 
of plants of one biotype, not necessarily the same one, at each 
station, although no biotype is completely eliminated (1g, 31, 65) 
In other words, a local inbreeding population has diverse biotypes 
available for response to changed environments that may occur 
in the future. 
Cooper (12) reported the results of a comprehensive ecological 
and genetical study on heading responses in local populations of 
cross-pollinating species of Lo/ium. He had shown earlier (11) 
that the flowering responses in these species are closely adapted 
to local conditions of temperature and day length. Cooper’s mate- 
rial satisfied Mather’s (47) two criteria: gene effects must be 
additive on the average and the non-heritable variance must be 
independent of the genotype. In each local population heading 
behavior is uniform under the conditions for which the population 
has been selected, but genetic diversity may be revealed under 
changed environmental conditions. 
These results support Mather’s statement that the population 
can possess high immediate fitness for its present environment 
and yet maintain a reserve of variability for evolutionary change. 
And as Cooper (12) wrote: “Such a population structure provides 
the genetic basis for local ecological adaptation.”’ 
Frandsen (16) pointed out that natural selection may perhaps 
act on the improved strains when grown in practical agriculture. 
Often soil conditions are better at the breeding stations. This and 
other favorable factors “may cause a shift in the genetical com- 
position of the improved strains due to natural selection, a fluc- 
