ANOMALIES OF DISTRIBUTION. 305 



like our own. Thus the question arises as to the cause of this 

 absence of Phyllopoda in countries where other forms can exist 

 which live tinder the same conditions, since both belong to the 

 animal group whose distribution on the globe seems to be caused 

 essentially by the action of winds. "NVe know that all these forms 

 produce eggs which can be dried without losing their power of 

 development, and that many actually require to have been dried 

 before the young can develope and escape. These eggs too are 

 minute and light, and certainly can be borne as dust before the 

 wind. \Vhy then are Apus and Branchipus absent in tropical 

 localities where the other Crustacea nevertheless occur ] This 

 striking anomaly is, however, as is easily shown, more apparent 

 than real. The establishment of such forms depends not merely 

 on the practicability of their germs being universally distributed 

 in the manner above indicated, but also on the favourable 

 accessories in the new conditions in which they find themselves. 

 Hence it would be very possible that the eggs of Apus and 

 Branchipus might reach the same tropical lands as those of the 

 other Crustacea, but not find there the circumstances that would 

 favour their development. What the hindering causes may be 

 it is difficult to say, for we are now only at the very threshold of 

 our knowledge of the vital conditions of these creatures ; but if 

 it were allowable to generalise from Brauer's elegant experiments 

 we might say that perhaps it is the absence of winter-cold 

 between the tropics which constitutes this hindrance, for he has 

 shown that the eggs of several Phyllopoda, at any rate, develope 

 most rapidly, or perhaps only, when they have previously been 

 exposed to a very low temperature, nearly down to the freezing 

 point. 



The very general uniformity of the lower forms of fresh- 

 water animals is, in the second place, interrupted by the occur- 

 rence, among numerous species of typical European character, of 

 isolated forms which appear perfectly foreign among their asso- 

 ciates. Thus, among many Kotatoria in the Philippines which 

 can hardly be specifically distinguished from the European 

 species, there are a few quite divergent forms. The most 

 remarkable of these is one named by me Trochosphcera cequa- 

 toris.ilis (see fig. 81) and which I described as long ago as 1872. 



