101 'Sni. C. DABWFN OJ.' TIIF SEXUAL RELATIONS OF 



tlio X. saJicfiria to grow by themselves, then if the species were 

 dimorphic it would only be an equal chance in fovour of the two 

 turning out different forms and consequently both being fertile ; 

 but as tlio species is trimorphic and each form can fertilize the 

 two other forms, it is two to one iu favour of the two turning out 

 different forms and being consequently both fertile. Wo thus 

 sec liow reciprocal trimorphisui must bo an advantage ; aud 

 probably it would be more advantageous to this Lythrum, whicli 

 commouly grows in almost a single row along the banks of 

 streams, than it would bo to Primroses or Cowslips which have 

 neighbours On all sides. But even if trimorphism etfected fio 

 good beyond that gained by dimorphism, we ought not to feel 

 mvich surprised at its occurrence, for we continually see throughout 

 nature the same end gained by the most complicp^tod as well as by 

 the most simple moans : to give one instance : — in maliy dioecious 

 plants pollen is carried from the male to the fem.ile 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 Catasetiim the conveyance 

 is effected by the most complex machinery ; for in this orchid we 

 have sensitive liorns which when touched cause a mombrano 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 viscid matter, and then bj^the breaking 

 of an elastic 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 eases, in fact depends on all the 

 previous stages through whicli 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 

 trimorphic plant like Lythrum salicaria could become dioecious 

 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 load to this end. As far as Lythrum salicaria is con- 

 cerned, the one tendency 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 docs the pollen 

 of the corrospoudiug stamens in the other two forms ; and this 



