2OO EVOLUTION OF TO-DAY. 



Summary. 



The present distribution of animals over the world 

 is the result of countless interacting factors both 

 present and past. The present distribution is the 

 outcome of the past, and the former cannot be un- 

 derstood without the latter. But unfortunately our 

 knowledge is largely confined to the present, for the 

 past distribution of species is almost unknown. 

 Fossils only serve to give us occasional glimpses of 

 the past, enough to make possible a fragmentary 

 history of the general past history, but of the 

 origin and distribution of species almost nothing is 

 definitely known. In the past as in the present 

 have occurred migrations to and fro in search of 

 food ; some of which we can get a few hints 

 of, while others are absolutely unknown. All 

 agencies operating to-day, with the exception of the 

 influence of man, must have had equal force in the 

 past. Every thing combines to make the subject 

 one of the greatest complexity, and to confine our 

 observations to general statements. 



Such general conclusions in reference to past 

 distribution as can be made, give great assistance 

 in interpreting the present, and make it possible to 

 create a science of the subject of geographical dis- 

 tribution. When the past history of a group can be 

 read with any approximation to exactness, its pres- 

 ent habitat becomes intelligible ; and it is even pos- 

 sible to make predictions as to the future discovery 

 of fossils in definite localities. The fact that during 

 all of their past migrations the species have been 

 constantly changing, indicates that we are dealing 



