MISCELLANEOUS PLANTS 



187 



Islands, on the Alacran Shoals, and in the Bermudas. That the 

 Bermudas received their representatives of the genus through the 

 currents is the opinion both of Hemsley and Millspaugh. Though 

 its capacity for dispersal by this agency is variable, as is shown in 

 the table below, it is significant that the species which has made its 

 home in Bermuda is the plant that may exhibit the greatest buoyancy 

 in its fruits. Yet in this there is a difficulty, since the drifting 

 passage from the West Indies to Bermuda would occupy ten or 

 twelve weeks (see Note 14 of the Appendix); whereas my experi- 

 ments indicate that under the most favourable conditions the fruits 

 of C. lanceolata would on the average float for only three or four weeks. 



A similar point is raised by the occurrence of Cakile maritima, the 

 European species, in Iceland. It is shown in my experiments that 

 its fruits are not suited for crossing a tract of sea much over 100 

 miles in width, whilst the Faroe Islands, the nearest land to the 

 eastward in the direction from which they must have come, are 

 almost 250 miles away, and a drifting passage against the prevailing 

 winds and currents is involved. It would be easier for fruits of 

 West Indian species of Cakile to reach Iceland in the Gulf Stream 

 drift so often cast up on its shores than for those of C. maritima to 

 traverse the distance that separates it from Europe, provided that 

 the fruits possessed in both cases the floating power. But the passage 

 from the West Indies would occupy many months, and the unfavour- 

 able climatic conditions would in themselves inhibit the establish- 

 ment of a West Indian plant. A clue may be offered when we 

 regard the associates of C. maritima on the Icelandic beaches, namely, 

 Armeria maritima, Glaux maritima, Lathyrus maritimus, Matricaria 

 maritima, Mertensia maritima and Silene maritima (see Babington 

 in Journ. Linn. Soc, XL, 1871). From the data for five out of these 

 six associates given in my book on Plant Dispersal, it is to be inferred 

 that only two of the plants, Lathyrus maritimus and Matricaria 

 maritima, would have reached Iceland by the currents. In the case 

 of Mertensia maritima, about which I possess no data, dispersal by 

 currents would seem most unlikely. Three or four of the associates, 

 which occur on both sides of the Atlantic and extend into Arctic 

 latitudes, raise quite other questions than those of dispersal. On 

 the whole I am inclined to hold that the Icelandic strand flora, in 

 which the genus Cakile takes a part, mainly reflects the effects of 

 past changes in circumpolar distribution and is but little concerned 

 with the currents. 



Taking the case of Cakile lanceolata as representing both potentially 

 and actually the limits of possibilities for the dispersal of the genus 

 by currents, we may, I think, accept the conclusion, which is war- 

 ranted by the facts given below, that currents may effectually dis- 

 tribute the plants along continental coasts and between islands 

 100 to 200 miles apart. Currents will not explain the existence of 

 the genus on both sides of the North Atlantic. We must look for 

 the explanation in a common centre of dispersion of the genus in 

 Arctic latitudes when warmer climatic conditions there prevailed. 

 In other words, we must appeal to the theory advocated by Dyer 

 which is discussed in Chapter XV. 



