1902.1 AND ANCIENT GEOGKAPHY. 393 



crabs arrived there presumably in a very recent period. Only the 

 boundary in Mexico needs investigation, but possibly here it is not 

 temperature that puts a stop to the northern advance of the crabs, 

 but another climatic factor, namely, the arid or semiarid character 

 of the country lying to the north of the actual boundary, which 

 possibly has existed from the beginning of the Tertiary. 



The above considerations would sufficiently explain the third, 

 fourth and partly the fifth points (see p. 390), namely, the absence 

 of crayfishes in Africa, the absence of Potamobiidcz in Central and 

 South America and their absence in South and Southeast Asia 

 and on the Malaysian islands. They could not enter Africa and 

 could not go beyond Mexico on account of the presence of crabs 

 in these parts, and in Southeastern Asia and Malaysia they must 

 have once existed, but, have succumbed under the onslaught of the 

 crabs. This latter cause seems also to control the distribution of 

 the crabs and crayfishes in Southern Europe (see p. 377). It does 

 not explain, however, the absence of crayfishes in Central Asia, 

 and, as regards this point, we are unable to offer any reasonable 

 explanation (see p. 378 and footnote). 



CONCLUSION. 



Although we have tried to advance explanations for many of the 

 puzzling facts in the distribution of the freshwater Decapods dis- 

 cussed here, we are to bear in mind that the ideas brought forward 

 are largely hypothetical and tentative. In many respects we have 

 found a wonderful agreement between the distributional facts and 

 what is known about the geology and tectonics of the respective 

 parts, and it was one of my chief purposes to point out that it is 

 possible to more closely correlate zoogeography and geology. But, 

 nevertheless, I am fully aware of the danger that lies in our incom- 

 plete knowledge, not only of the geological configuration of the 

 different countries here discussed, but also in the deficiency of the 

 chorological facts at hand. 



I wish most strongly to emphasize that I do not believe in all 

 cases to have correctly revealed the ancient relations of land and 

 water, and I think that my ideas of the old continents need con- 

 firmation and probably modification. I have only tried to give a 

 representation of what I think of the changes that have taken place 

 during the earth's history, as tar as the present state of our knowl- 

 edge permits of any conclusions in this respect, and I earnestly 



