xvi The Botanical Work of Darwin. 
example of his persistence; they began to interest him in 
about 1862, and he was still hoping to attack them in 1881, as 
illustrated in letters to Sir W. Thiselton-Dyer and others, 
which I hope soon to publish. 
His next great work in the domain of fertilization was his 
papers onHeterostylism, the well-known researches on Primula, 
Linum ) Ly thrum , &c., which were afterwards worked up with 
other cognate matter into his book on Forms of Flowers, 1877. 
This work was extremely laborious, but with the labour went 
an especial delight in reading the riddle. Thus he wrote : — 
‘ I do not think anything in my scientific life has given me so 
much satisfaction as making out the meaning of the structure 
of these plants V While the delight of unravelling a biological 
puzzle was, as usual with him, the chief incentive to work, he 
did not overlook any fraction of the lessons to be learned from 
his results. In the case of heterostylism, he thought that the 
essential value of the problem lay in its bearing on hybridism. 
The parallelism between hybridization and certain heterostyled 
unions is wonderfully close, so that it is hardly an exaggeration 
to say that ‘illegitimately ’ reared seedlings are hybrids between 
members of a single species. This fact gave a death-blow to 
the doctrine (which has been so long in dying) that differences 
in sexual constitution are ‘ the very touchstone of specific 
distinction V 
Another group of his researches was connected with the 
irritability of plants as exhibited in movement. The earliest 
of these was his work on Climbing Plants, 1865, in which he 
followed Mohl and Palm, but with the addition of so much 
new matter that he practically made the subject his own. 
In his Insectivorous Plants, 1875, he also discovered and 
thoroughly investigated a number of remarkable instances of 
plant movement. His account of the sensitiveness of Drosera 
to excessively minute weights, though since shown by Pfefifer 
to bear a somewhat different interpretation, yet remains one 
of the most wonderful instances of vegetable irritability. 
1 Life and Letters, Vol. i, p. 91. 
2 Forms of Flowers, p. 276. 
