786 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



capacity for overseas dispersal. The most notable advance in our understanding 

 of such distributions has come with the appreciation of the importance of aerial 

 dispersal by winds. The fact, for example, that the Hawaiian spider fauna con- 

 sists exclusively of those families that disperse by means of gossamer flights is 

 essentially conclusive as to their wind-blown origin. The complete renewed 

 analysis of the Hawaiian fauna by Elwood C. Zimmerman confirms the dishar- 

 monic nature of that fauna already evident in the earlier Fauna Hawaiiensis 

 of R. C. L. Perkins. In his introductory volume for Insects of Hawaii (1948) 

 Zimmerman presents a masterly and concliisive review of the problem of origin 

 of the fauna. He finds it to be undoubtedly the growth of ages of flotsam- jetsam 

 overseas immigration combined with wind-blown insects and accidental arrivals 

 of strayed and off-course land birds. 



The West Indies have been a classical meeting ground for speculations as to 

 the origin of their fauna and as to possible land connections with Central and 

 South America. The much more radical idea that they had been directly con- 

 nected by land with the Mediterranean region, via North Africa, as in Joleaud's 

 llipparioyi bridge, crops up repeatedly. This is adopted by Jeannel to account 

 for the presence of certain carabid beetles in the West Indies, namely of species 

 whose congeners are found in the Atlantic Islands and North Africa or in the Old 

 World generally. P. J. Darlington, Jr. (1938) has brought this problem into 

 focus as a problem of dispersal; pointing out that the beetles cited by Jeannel as 

 indicating land connection are among the smallest members of the carabid beetle 

 fauna of the West Indies, which enormously increases the probability of their 

 aerial dispersal. When the prevailing winds are plotted, and hurricanes and 

 their tracks considered, the probabilities of arrival of these isolated elements of 

 the West Indian fauna by aerial dispersal become overwhelmingly great. 



Darlington's discussion of aerial dispersal of insects is a return to Darwinian 

 thinking about the problem of dispersal in general. A notable contribution to the 

 problem of the possibilties and probabilities of aerial dispersal is supplied by 

 direct studies of the objects found in the air by airplanes. The paper by P. A. 

 (Hick, "The Distribution of Insects, Spiders, and Mites in the Air," published 

 in 1939, summarizes data on collections made by means of special traps placed on 

 airplane wings. More than 30,000 specimens of eighteen orders of insects, plus 

 the spiders and mites, were obtained at altitudes ranging from 200 to 15,000 

 feet, the highest altitude being represented by a single specimen of spider. These 

 observations establish beyond doubt the possibility of dispersal of small creatures 

 of all kinds through the air. The most recent summary of the data of this kind is 

 by Gislen, in 1948. 



Life of Fresh Waters 



Aerial dispersal had long before been shown to be the explanation of the 

 strikingly wide distributions of freshwater organisms. Bodies of fresh water are 

 usually sharply isolated, and by analogy with isolation on land, their life might 

 be expected to exhibit great corresponding differences from lake to lake and 

 from river to river, which is not found to be the case. The English geologist 

 Thomas Belt further amplified the simple explanation of the wide uniformity of 

 freshwater life offered by Darwin. He was much impressed, in Nicaragua, as 



