EXPERIMENTAL STUDY OF HEREDITY 551 
1. "The experimental plants must necessarily possess 
constant differentiating characters." 1 
2. "The hybrids of such plants must, during the flower- 
ing period, be protected from the influence of all foreign 
pollen, or be easily capable of such protection." 
3. "The hybrids and their offspring should suffer no 
marked disturbance in their fertility in the successive 
generations." 
Mendel also called attention to the advantage of choos- 
ing plants which, like the peas, are easy to cultivate in 
the open ground and in pots, and which have a relatively 
short period of growth. 
472. Characters Chosen for Observation. "Each pair 
of differentiating characters [have been thought to] unite 
in the hybrid to form a new character, which in the pro- 
geny of the hybrid is usually variable. The object of the 
experiment was to observe these variations in the case of each 
pair of differentiating characters, and to deduce the law ac- 
cording to which they appear in successive generations. The 
experiment resolves itself therefore into just as many 
separate experiments as there are constantly differentia- 
ting characters presented in the experimental plants." 
The following were the characters chosen for observation: 
i. The difference in the shape of the ripe seeds (round 
and smooth vs. angular and wrinkled). 
1 Differentiating characters are those in respect to which the two species 
or varieties to be crossed differ. The possession of chlorophyll by the leaves 
of peas, for example, is a common character. "Common characters are 
transmitted unchanged to the hybrids and their progeny." The color of 
the corolla (for example, white in one species and purple in the other) is a 
di/erentiating character, serving to differentiate or distinguish one species 
from another. 
