Some Recent Work on Hybrids in Plants . 75 
Correns and Tschermak at about the same time. Mendel invest¬ 
igated the behaviour of the hybrids between the various races of 
Pisuni sativum his was the first thorough statistical study of 
hybrids, and as his methods of work were so convincing, his results 
so striking, and of such general importance that they open up wide 
avenues for further work, his observations are certainly worthy of 
close attention. 
Peas were selected by Mendel for his experiments, because 
they are easily protected from foreign pollen, are naturally self- 
fertilised, and because the hybrids are quite fertile, and so can 
easily be raised in large numbers though numerous generations. 
He crossed only races which had constantly differentiating pairs of 
characters, and he selected the following seven pairs of characters 
for investigation, (1) form of ripe seed, whether round and slightly 
wrinkled or irregular and deeply wrinkled, (2) colour of cotyledons, 
whether yellow or green, ( 3 ) colour of seed-coat, whether grey (or 
grey brown) or white, ( 4 ) form of pod, whether inflated or con. 
stricted, ( 5 ) colour of unripe pods, whether light to dark green or 
vivid yellow, (6) position of flowers, whether borne laterally on the 
axis or inaterminal cluster, ( 7 ) length of stem, whether long ( 6 - 7 feet) 
or short (f-1^ feet). 
The first striking point in Mendel’s work was that the hybrids 
produced by any of the crosses exhibited none of the above 
characters in an intermediate form, but in all cases one or other of 
the differentiating characters of any pair was transmitted 
unchanged, and all the hybrids followed one character. This 
character Mendel calls the dominant one, while to the other he 
applies the term recessive . In the list given above of the pairs of 
characters investigated those found by Mendel to be dominant in 
nature are placed first, the recessive second. Thus the hybrids, 
whatever their origin, all showed a seed of round form with shallow 
depressions, they were never of irregular or intermediate characters ; 
all the cotyledons were yellow in colour, never green or inter¬ 
mediate ; the seed was always grey (or grey-brown), etc., etc. 
The dominance of the special characters was very obvious in the 
hybrids themselves, but the great value of Mendel’s work lies in the 
fact that he made clear the exact behaviour of the differentiating 
characters in futher generations produced from the hybrids by self 
fertilisation. This he investigated not only in the simplest case in 
which hybrids were produced from races that differed in only one 
pair of characters, but also in more complex cases where the 
crossed races differed in several pairs of characters. 
