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 306 THE INFUTENCE OF INANIMATE SURROUNDINGS. 



I found this genus exclusively in Miixdanao. What is the reason 

 that it is absent from Luzon and Bohol, where the same external 

 conditions of existence would seem to prevail as on the southern 

 island? It is impossible to suppose that the eggs have not 

 been able to find their way thither ; but what the numerous 

 causes may be which affect their development, in one place 

 favouring and in another preventing it, is not at present known. 

 A second example of the same kind Ls offered by the forms 

 of Branchipus. Species of this genus live both in Europe and 

 America. The species occurring in the two continents, though 

 easy to distinguish, still are so similar that the American 

 species might be transferred to Europe and vice vemd without 

 changing anything in the character of the fauna of either coun- 

 try. But, associated with them, live a few other veiy divergent- 

 forms, particularly the very singular Thamnocephalus, which. 



FiQ. 81. — TrcchosphtEra cquatcrialis, a Philippine species (Rotatoria). 



disturbs the uniformity of the American Branchipoda by its 

 occurrence in the south of the Union. Here also the causes are 

 perfectly unknown which prevent this genus from developing 

 fully in thp higher latitudes of l^orth America ; but we are 

 obliged to assume that there are such hindrances, since it is 

 difficult otherwise to see why they should not develope in the 

 north just as well as the eggs of the other Branchipoda which 

 are distributed with great uniformity over the whole continent, 

 and which everywhere develope in the same manner. It 

 appears to me that Brauer's researches — so often alluded to — 

 if not as yet fully available, contain the germ of future and 

 more fertile inquiry in this direction, and it is only to be wished 

 that Brauer may not long remain in sole possession of this field, 

 for a combination of forces will in this, as in every case lead 

 sooner and more certainly to the desired result. 



