XII GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 365 



find that several are from twice to three times the weight 

 of the grains found by Mr. Murray, and others five times, 

 eight times, and fifteen times as heavy ; but they are pro- 

 portionately very much larger, and, being usually irreg-ular in 

 shape or compressed, they expose a very much larger surface to 

 the air. The surface is often rough, and several have dilated 

 margins or tailed appendages, increasing friction and rendering 

 the uniform rate of falling through still air immensely less 

 than in the case of the smooth, rounded, solid quartz grains. 

 With these advantages it is a moderate estimate that seeds 

 ten times the weight of the quartz gTains could be carried 

 quite as far through the air by a violent gale and under the 

 most favourable conditions. These limits will include five 

 of the seeds here given, as well as hundreds of others which do 

 not exceed them in weight ; and to these we may add some 

 larger seeds which have other favourable characteristics, as is 

 the case with numbers 11-13, which, though very much larger 

 than the rest, are so formed as in all probability to be still more 

 easily carried great distances by a gale of w4nd. It appears, 

 therefore, to be absolutely certain that every autumnal gale 

 capable of conveying solid mineral particles to great distances, 

 must also carry numbers of small seeds at least as far ; and if 

 this is so, the wind alone will foim one of the most effective 

 agents in the dispersal of plants. 



Hitherto this mode of conveyance, as applying to the 

 transmission of seeds for great distances across the ocean, has 

 been rejected by botanists, for two reasons. In the first place, 

 there is said to be no direct evidence of such conveyance ; and, 

 secondly, the peculiar plants of remote oceanic islands do not 

 appear to have seeds specially adapted for aerial transmission. 

 I will consider briefly each of these objections. 



Objection to the Theory of Wind-Dispersal. 



To obtain direct evidence of the transmission of such 

 minute and perishable objects, which do not exist in great 

 quantities, and are probably carried to the greatest distances 

 but rarely and as single specimens, is extremely difficult. A 

 bird or insect can be seen if it comes on board ship, but who 

 would ever detect the seeds of Mimulus or Orchis even if a 

 score of them fell on a ship's deck 1 Yet if but one such seed 



