194 MR. C. DARWIN ON THE SEXUAL RELATIONS OF 
the L. salicaria to grow by themselves, then if the species were 
dimorphic it would only be an equal chance in favour of the two 
turning out different forms and consequently both being fertile ; 
but as the species is trimorphie and each form can fertilize the 
two other forms, it is two to one in favour of the two turning out 
different forms and being consequently both fertile. We thus 
see how reciprocal trimorphism must be an advantage; and 
probably it would be more advantageous to this Lythrum, which 
commonly grows in almost a single row along the banks of 
streams, than it would be to Primroses or Cowslips which have 
neighbours on all sides. But even if trimorphism effected no 
good beyond that gained by dimorphism, we ought not to feel 
much surprised at its occurrence, for we continually see throughout 
nature the same end gained by the most complicated as well as by 
the most simple means: to give one instance :—in many dicecious 
plants pollen is carried from the male to the female by the wind, 
which is perhaps the simplest method conceivable, or by the 
adherence of the grains to the hairy bodies of insects, which is a 
method only a little less simple; but in Catasetum the conveyance 
is effected by the most complex machinery ; for in this orchid we 
have sensitive horns which when touched eause a membrane to 
rupture, and this sets free certain springs by which the pollen- 
masses are shot forth like an arrow, and they adhere to the 
insect’s body by a peculiar viseid matter, and then by the breaking 
of an elastie thread of the right strength the pollen is left sticking 
to the stigma of the female plant. The complexity of the means 
used in this and in many other cases, in fact depends on all the 
previous stages through which the species has passed, and on the 
successive adaptations of each part during each stage to changed 
conditions of life. 
As some authors consider reciprocal dimorphism to be the first 
step towards diceciousness, the difficulty of understanding how a 
trimorphie plant like Zythrum salicaria could become dicecious 
should be noticed; and as dimorphism and trimorphism are so 
closely allied, it is not probable that either state is necessarily 
in any way related to a separation of the sexes—though it may 
occasionally lead to this end. As far as Lythrum salicaria is con- 
cerned, the one teridency which we can discover is towards the 
abortion of the two sets of stamens in the mid-styled form. This 
tendency is evinced by its pollen, though abundant and apparently 
good, yielding a smaller percentage of seed than does the pollen 
of the corresponding stamens in the other two forms; and this 
