192 THE POPULAR SCIENCE MONTHLY 
true to its type because of self-pollination, and during the growing 
season the plants can be compared and any desirable type selected for 
future propagation. In a cross-pollinated plant like maize this is not 
the case. The pollen is carried by the wind through long distances and 
varieties planted close together are continually intercrossed. ‘The 1so- 
lation of a particular type is not simple as in the case of wheat, but may 
be prolonged through many generations. Hach prize ear selected for 
future planting will have had at least a few and possibly many of its 
seeds fertilized by pollen from less desirable strains. When these seeds 
are grown they of course again fertilize the seeds of the desirable plants 
with a frequency proportionate to their number. 
In the case just cited recourse may be had to artificial self-pollina- 
tion. Several hundred seeds are thus produced at one operation and the 
work of isolating the new variety is made materially easier. But sup- 
pose we are dealing with red clover where the flowers are small, almost 
sterile with their own pollen and produce only one seed. In this crop, 
the long and tedious method of continuous selection just mentioned 
must be used, for there is no other way. This method is often called 
the pedigree-culture method. The main idea of the plan is that the 
seeds of single plants are grown in isolated plots, and the character of 
the mother plant judged by the characters of the progeny. This 
method has given much better results than the so-called German 
method, which consisted in planting a mixed lot of seeds from several 
of the best plants. For example, the German sugar-beet raisers have _ 
for years analyzed large numbers of sugar-beets and have grown their 
seed from the mother beets showing the highest percentage of sugar. 
No particular attention was paid to the general average of the progeny 
of each beet; those were bred from which appeared to be the best as 
shown by the polariscope sugar test. In this way the amount of sugar 
produced per acre was gradually increased, but progress was slow and 
cessation of selection immediately caused the sugar content to decline. 
To see the real reason of this we must go back to the time of Dar- 
win. ‘The data from which Darwin proved the doctrine of descent — 
came in large measure from domestic animals and cultivated plants. 
He saw that plants varied among themselves and that by selection of 
the variants new types were gradually produced. From these facts he 
argued that all evolution had taken place by the selection of minute 
variations and generally through the selective agency of a contest for 
life taking place among all living organisms. This he called the agency 
of natural selection. Later, however, Bateson, Korshinsky and de Vries 
called attention to the fact that many new types of animals and plants 
are known to have originated suddenly. There was no gradual evolu- 
tion of the type; it simply appeared fully formed. This hypothesis, 
called the “ mutation theory,” found great favor among plant breeders 
