402 THE AMERICAN NATURALIST [Vol. LI 



the result of such an importation. At the utmost, the re- 

 sult will be, that the rat population of the warehouses in 

 the port of entry will become somewhat more variable. It 

 may happen that a single warehouse, empty of goods and 

 rats, will receive a small colony of imported rats with a 

 load of rice and rattan, but on the type of the rats of Java 

 such an occurrence will be of no importance. 



A very different thing must happen, when rats from 

 ships come ashore in places where there is as yet no popu- 

 lation of rats of that same group, for instance, on newly 

 settled islands. There the imported rat population will 

 gradually become constant, but as often for an own, new 

 set of characters, as for those of one of the species, which 

 originally went into the composition of the ship's popu- 

 lation. The rat population of the bigger ships very often 

 is a very peculiar one. It is not uncommon to find a very 

 homogeneous lot of rats on board a ship, for which no 

 corresponding tyi>e specimen can be found in any mu- 

 seum. In other instances the population of a ship may be 

 very heterogeneous indeed. The rat population of a ship 

 originates from rats which come aboard with cargo in 

 the most diverse places. By crossing of such animals, all 

 kinds of new types may arise. The rats on board a ship 

 live under very peculiar circumstances. As the animals 

 can not emigrate, their number is absolutely dependent 

 upon the kind of goods stowed in the ship. For a time the 

 circumstances may be so favorable for a multiplication 

 of the animals, that the ship is speedily overrun with rats. 

 Especially is this the case when part of the load affords 

 good hiding places, such as rattan bundles, and if food is 

 abundant, as in a load of copra. On unloading part of 

 the cargo, a famine may result, from the effect of which 

 all the animals, excepting only a very few, may succumb. 



The result of such a catastrophe, especially of a series 

 of catastrophes, alternating with periods of plenty, must 

 be, that the population, no matter how variable at one 

 time, must very quickly become pure for a genotype of 

 its own. The occurrence of rats on board of ships is so 



