Robertson: Theortes of Evolution. 243 
two different races of peas were chosen for the experiment. 
First of all the seeds of all these strains were grown separately. 
for two years to make sure that they were genuine races and 
not mixtures. They were found to come quite true and con-. 
stant from seed. In crossing his peas, Mendel concentrated his 
attention on certain definite differences between the races. He 
decided to investigate the inheritance of sharply defined pairs of 
characters by the possession of one or other of which the 
different races could be distinguished. These were :— 
1. Difference in form of ripe seeds. Round or wrinkled. 
2 oF ,, colour of cotyledons. Yellow or green. 
Zn " ,, colour of seed-coats. Coloured or white. 
4. at ,, form of ripe pods. Inflated or constricted. 
5: 3 ,, colour of unripe pods. Green or yellow. 
6. r ,, position of flowers. Axial Or terminal. 
a: 5 ,, length of stems. Tall or dwarf. 
A set of experiments was made with each of these pairs of 
differentiating characters. For example, in the first set of 
experiments plants bearing round seeds were crossed with 
plants bearing wrinkled seeds, and in the second set plants 
with yellow cotyledons were crossed with plants with green 
cotyledons, and so on. Seven sets of hybrid plants were thus 
produced. It might have been supposed that these hybrids 
would turn out to be intermediate in their characters between 
the parents (for instance, that the peas resulting from a cross 
between a round pea and a wrinkled pea would be intermediate 
in form between the two), but it was found that this was not 
the case. All the hybrid peas were round—no wrinkled ones 
were found at all. The same results were got with the other 
six pairs of differentiating characters—in each case one of the 
two characters had got the upper hand, and the other did not 
appear at all; the offspring of the tall peas and the dwarf peas 
were all tall—the offspring of the.peas with inflated pods and 
the peas with constricted pods were all inflated, and so on. It 
is convenient to distinguish the two opposing characters of each 
pair respectively as domznant (D) and recesszve (R), the dominant 
character being that which appears in the hybrid to the exclusion 
of the recessive character which remains latent. On the hybrids 
being self-fertilised and their offspring again examined, a most 
curious result came to light. I will only describe the case 
of the round and wrinkled peas, as the other cases fall exactly 
into line with this. The first generation of hybrid peas were 
1997 July 1. 
