EXPERIMENTAL EVOLUTION 
521 
photographs and written descriptions, and by preserving 
dried and pressed herbarium specimens. The plants of 
the second generation were carefully guarded from being 
cross-pollinated, and thus "pure" seed were secured for a 
third generation. This was continued often for 25 or 30 
generations of the plant, requiring as many years when a 
FIG. 390. Hugo de Vries. His pioneer studies of osmosis resulted in 
fundamental contributions to our knowledge of that subject; his mutation- 
theory is one of the most important contributions to the study of evolu- 
tion since Darwin. 
species produced only one crop of seed a year. Very care- 
ful records and preserved specimens were kept of the plants 
of each generation, and accurate comparisons were made 
to see if any individuals showed a tendency to vary widely 
from their parents in any significant way, such as showing 
entirely new characters, not expressed in the parents, or 
failing to manifest one or more of the characters of the 
parent. 
