384 



THE AMERICAS' S'ATURALIST 



[Vol. L 



I take the liberty of referring to my paper on the spiders of the 

 Greater Antilles. 6 On pages 139 and 140 it is shown that sixteen 

 Antillean genera, selected in a rather random fashion, are found 

 to-day in regions connected by a certain hypothetical system of 

 trans-oceanic bridges and not elsewhere. In some cases there 

 appears to be almost no specific difference, even, between spiders 

 whose present range is far from continuous. In fact, this set of 

 data and other instances mentioned in that paper seem to me to 

 furnish as strong evidence for such bridges as is given by the 

 rtvent distribution of any one group of organisms, and yet I felt 

 that this was not at all the explanation. It seemed more rea- 

 sonable to believe that spiders had dispersed by the way of 

 Holarctica on land masses which were practically the same as 

 they are now and that the present discontinuous distribution of 

 the ancient types is brought about by the fact that they are 

 merely relicts which are now found far separated from each 

 other. 



Entomologists will at once think of a number of species which 

 have reached the United States in historic times from Mexico and 

 many will use these in a contention that much of our fauna has 

 been derived from the south. I believe that the movement has 

 been largely the other way and that the "southern element" of 

 our fauna is largely made up of those things which have dropped 

 behind in the general southern movement. As in any stream 

 there are back eddies, so in the stream of dispersal we must ex- 

 pect back eddies (especially when man makes a channel as he did 

 when he planted a large number of; potatoes up to the former 

 habitat of the Colorado potato beetle or grew great quantities of 

 corn and other cereals for the chinch bug) but the eddies do not 

 indicate the direction of the main stream. In their progress 

 toward the equatorial region the streams of dispersal leave pools 

 here and there— the stranded relicts of an ancient fauna. There 

 are doubtless numerous swirls and back currents, while near their 

 mouths these streams of dispersal may be much subject to "tides" 

 due to minor, i. e., measured by centuries, fluctuations of climate, 

 but their general movement is, nevertheless, from the poles 

 toward the equator. Since the northern polar regions have the 

 larger land masses and better facilities for such dispersal-stream 

 flow, the larger movements have been from the north, but simliar, 

 though smaller, currents are to be expected in the southern 

 hemisphere. ±" rank E. Lutz 



