W. F. R. Weldon 
229 
I. — Mendel's Results with Peas. 
In 1865 Gregor Mendel (No. 21) described the results of crossing various 
races of the common Pea. He does not waste time in discussing the question 
whether all his races belong to one " species " or not, but describes the result of 
crossing any two of them as " hybrid." It is probable, however, that all his races 
belonged to what is now called Pisura sativum, to the exclusion of Pisum arvense. 
The races used differed, no doubt, in many ways: but special attention is paid 
to seven sets of characters, with regard to each of which it was possible to sepa- 
rate the races into two categories. Thus the shape of the seeds might be round, 
with only slight and shallow wrinkles on the surface, or irregular and deeply 
wrinkled. The cotyledons of the seeds might be yellow or green in colour, and 
so on. The pairs of characters, recognised in this way for each organ or set of 
organs studied, are distinguished, according to their power of affecting hybrid 
offspring, into dominant and recessive. The characters presented by a race are not 
necessarily all " dominant," or all " recessive " ; thus the character of roundness in 
seeds is dominant, that of wrinkled irregular shape recessive ; yellow colour of 
cotyledons is dominant, and green recessive ; but a race of peas may have smooth 
and rounded seeds with green cotyledons, or yellow cotyledons and wrinkled 
seeds. 
The first general result obtained by Mendel may be stated as follows : If peas 
of two races he crossed, the hybrid offspring will exhibit only the dominant cliaracters 
of the parents ; and it will exhibit these witltuat (or almost without) alteration, the 
recessive characters being altogether absent, or present in so slight a degree that they 
escape notice. 
This may be called the Law of Dominance, and it at once explains the terms 
dominant and recessive. 
The second result is that ; If the hybrids of the first generation, produced by 
crossing two races of peas which differ in certain characters, be allowed to fertilise 
themselves, all possible combinations of the ancestral race-characters will appear in 
the second gerieration with equal frequency, and these combinations will obey the 
Law of Dominance, so that characters intermediate between those of the ancestral 
races will not occur. 
From its consequences, this may be called the Law of Segregation. 
The significance of these results may most easily be seen by considering an 
example. It has been said that yellowness of cotyledons is dominant over 
greenness of cotyledons. That is to say, if a plant from a race with yellow 
cotyledons and one from a race with green cotyledons be cross-fertilised, the 
resulting seeds will have yellow cotyledons, no matter which plant be used as the 
female parent. Mendel chose ten plants, some from a race with green and some 
from a race with yellow cotyledons, and upon these plants he made 58 crosses, 
so that the $ parent was sometimes of green-seeded, sometimes of yellow-seeded 
race. The resulting seeds had always yellow cotyledons. From these seeds 
