PROBLEMS AND RELATIONS 9 



from the optimum. The study of adaptations is for this reason of 

 especial importance to an understanding of the ecology of animal 

 distribution. 



Every process of this kind can be verified experimentally and may 

 be made the subject of physiological analysis. We are, to be sure, 

 still at the beginning of an experimental ecology, and zoologists are 

 less advanced than the botanists in this field. This branch of knowl- 

 edge is certain to undergo active development, on account of the 

 abundance of interesting results promised. Laboratory studies have 

 been made of the change of form in daphnias under the influence of 

 food and temperature ; of the transformation of the salt-tolerant crus- 

 tacean, Artemia salina, with changes in the salinity of the water; and 

 of the relations between frogs and their water supply. Field experi- 

 ments have also yielded important results; species of turbellarians 

 have been introduced in the brooks of the island of Riigen, where they 

 were absent, though present in the mainland brooks; a Danish race of 

 Daphnia cucullata was introduced in Lake Nemi in the Albanian 

 mountains; special races of Peridinia and of Anodonta have been 

 introduced into newly made artificial ponds in order to study the 

 ensuing changes. 



Unplanned experiments have been still more numerous, and their 

 results throw light on certain questions of ecological zoogeography. 

 The whitefish introduced into Lake Laach (near Coblenz) have trans- 

 formed into a new subspecies {Coregonus fera benedicti) . The intro- 

 duction of English foxes into Australia, and of the muskrat into 

 Europe, are further examples from the very long list of such natural 

 experiments. The release of the English sparrow and of the European 

 starling into North America (by the same person!) are familiar Amer- 

 ican examples. The great series of foreign insect pests brought into 

 the United States have been much studied. The most illuminating and 

 comprehensive work on this subject is Thomson's The Naturalization 

 of Animals and Plants in New Zealand. The possibility of an experi- 

 mental study of the problem of ecological zoogeography gives to the 

 results obtained a potential degree of certainty, quite in contrast 

 with the necessary uncertainty of the conclusions of historical 

 zoogeography. 



The aims of historical zoogeography are unquestionably high, and 

 valid answers to its questions would be of great importance. It is 

 remarkable that one is able to find clues to events of the remote 

 past by the analysis of the homologies in animal distribution. His- 

 torical zoogeography has been valued highly in the eyes of numerous 

 investigators in the endeavor "to unravel the history of the coloniza- 



