88 Treubia Vol. Ill, i 



But still there is a possibility that something new may be found on 

 one of these islands, a form confined to it and not existing on the. other 

 islands. 



First of all such a form may originate by environmental changes A certain 

 form having invaded Krakatau, or one of the other islands, may find 

 the conditions there different from those it is used to. The new environ- 

 mental conditions can effect a constant aberration of the original form 

 as long as these new conditions last. On Krakatau and Verlaten L the 

 flora is still far from normal, and the abnormal vegetation perhaps gives 

 rise to new forms, but in future the forest will become less and less 

 different from that on other islands. 



But also other conditions may be of influence, for instance the absence 

 of fresh water on Krakatau or the different food animals find there. 



It will be also important to see how the newly introduced house rats 

 will behave in future. If Krakatau remains uninhabited and uncultivated, 

 these rats will find there quite other conditions than ordinarily, accustomed 

 as they are to live in houses, to nest especially in the upper half of 

 buildings, and to take for food the easily accessible stores in or around 

 dwellings. There is a chance this subspecies will revert then to the original 

 forest-dwelling species of rat, from which both house rats and country rats 

 have originated. 



Another mode of origin of new subspecies or races is by isolation. 

 A species spread over a large area of land with uniform local conditions, 

 and having a tendency to differentiation, is as a rule a mass of forms, a 

 certain "population" ; only it is impossible to separate these forms from 

 each other except perhaps by breeding. Now new land is not invaded 

 by such a species as a whole, but more probably by some straggling 

 individuals. These individuals are likely to lack some characters of the 

 species or to exhibit some others; if they remain isolated these forms 

 become more and more fixed and may be easily separated then from the 

 species of which they are offsprings. 



The third possibility is that a new form comes into existence by the 

 hybridization of two or more species or subspecies invading the island at 

 the same time or shortly after each other. 



All the above mentioned modes may be combined, each with one or 

 two of the others. 



Recently Mr. MOULTON has been describing two new Danaines from 

 Krakatau and Verlaten I., viz. Danaida javenta Cr. krakatauae and D. 

 melanippiis Cr. insiUaris, the first subspecies from Krakatau and Verlaten I., 

 the second from Krakatau only. Of D. //z. //zs/z/ßm Mr. MoULTON writes: — 

 "This Krakatau form suggests a possible hybrid between hegesippus and 

 melanippus. Possibly stragglers of melanippus from Java and hegesippus 

 from Sumatra have reached Krakatau since the great eruption and have 

 given rise to this new race." 



