408 The zoologist. 



genera. These generic maps could be easily compiled from 

 the specific maps, when completed. They would probably 

 throw some light on the value of the generic distinctions which 

 have been based mainly on anatomical structure. In the case of 

 these maps, it would be sufficient to indicate by means of one 

 colour the entire area covered by a genus as represented by 

 any of its species, either in summer or winter. The distinction 

 between the haunts of the different species belonging to the 

 genus in question at different times of the year is, for this 

 purpose, of no importance whatever. Similar maps, showing the 

 distribution of families and orders, might also be prepared, and 

 would certainly prove interesting. 



6. Key Maps. — In addition to the foregoing maps it is 

 desirable (as already stated) to have certain other maps which may 

 be called " Key Maps," representing the various chief natural 

 forces and physical facts which have any effect on the distribution 

 of animals, or which govern their migrations. They should, in 

 the first instance, be plain uncoloured maps, exactly similar to 

 those used in marking out distribution, but they should be printed 

 on some transparent paper, such as tracing-paper. They should 

 ultimately, for use, be coloured by hand, in such a manner that 

 each one shows the existence and influence over the surface of 

 the world of some one physical or natural force. Thus, one 

 key map would indicate the elevation of the surface ; another, 

 the prevailing ocean currents ; another, ocean depths and ancient 

 coast lines ; others, the prevailing wind currents at different 

 times of the year ; others, the chief mountain ranges and river 

 systems of the world, rainfall, slope exposure, distribution of 

 forest, hygrometry, geological formations, &c, &c. When so 

 coloured, these maps might be taken in turn and laid over 

 each completed map showing the distribution of any particular 

 species. By this means, a more or less clear idea might be 

 obtained of the effect which the various physical facts and causes 

 shown on the different key maps have had on the distribution of 

 the species in question ; for the distribution of the species would 

 be seen through the transparent maps showing the various 

 physical forces, and a careful comparative examination would 

 probably to some extent show the correlation of the two, or, in 

 other words, what effect the latter had upon the former. 



