244 Robertson : Theories of Evolution. 
all round, but when these were sown and the flowers of the 
next generation self-fertilised, a mixture of smooth and wrinkled 
peas were obtained. There were no intermediates, and usually 
some of the peas in the same pod were round and some wrinkled. 
From 253 hybrid pea plants 7,324 seeds were obtained, of which 
5,474 were round and 1,850 were wrinkled, z.e., there were almost 
exactly three times as many round peas as wrinkled peas 
(2.96:1). This proportion was found to hold good for each ' 
of the seven pairs of characters worked with; on an average 
the proportion in the seven trials was 2.98 : 1, which is 
practically 3 : 1. 
To understand what these numbers mean we must follow 
the hybrids to further generations. Take first the one quarter 
which show the recessive character, e.g., the wrinkled peas. 
These, when self-fertilised for a number of generations, continue 
to produce nothing but wrinkled peas, so that they may be 
regarded as a pure wrinkled race. But the remaining three- 
quarters which show the dominant character, roundness, when 
self-fertilised, do not all behave alike ; they break up into round 
and wrinkled peas in a constant numerical proportion. 
Mendel required some theory to account for these remarkable 
numerical relations, and he put forward one which is extremely 
simple and complete. He supposes that ¢he germ cells are pure 
with respect to certain characters, that is to say, in the case of 
two opposing characters, such as roundness and wrinkledness 
of seeds, each egg and sperm can only transmit one or other 
attribute, and not both. He makes also the further postulate 
that on an average one half of the germ cells carry one 
character and one half the other character. The hybrid plants 
produced by crossing a round pea with a wrinkled pea would 
on this hypothesis bear eggs, half of which carry the character 
of roundness and half of wrinkledness, and in the same way 
sperm cells, half of which carry each character. When the 
hybrid is self-fertilised the eggs and sperms mate according to 
the laws of chance. Let us consider the case of four eggs and 
four sperms mating in this way :— 
EaGs. SPERMS. 
De—_____———+5D 
ene 
As the result of this mating we get 
rDD; 2DR, TRR; 
Naturalist. ; 
