GEOGRAPHICAL DISTRIBUTION OP VERTEBRATE ANIMALS. 407 



certain areas were during the winter or the summer; if during both, 

 asterisks of both colours would, of course, be used. In the case of isolated 

 occurrences of a species beyond its known range, it would sometimes be 

 difficult to decide whether it should be entered as in summer or in winter. 

 The time of year would often be an inefficient guide, so that in each 

 individual case probabilities alone could be relied upon. 



(g). Uncertainty as to the boundaries of the area occupied by any species, 

 in cases where, although there are no actual records, it probably occurs 

 over a wider district than is proved to be the case, may be indicated by 

 marks of interrogation placed close together, in regular pattern covering 

 the region in question as in the case of the asterisks above mentioned ; the 

 colour, of course, indicating whether the species may be expected to inhabit 

 the conjectural area during the summer or the winter. For instance, 

 in the case of two or more adjacent, but disconnected, areas being shown 

 for one species, without there being any apparent cause for, or actual 

 information respecting, its absence in the intervening space or spaces, 

 it is a reasonable conjecture that the species in question really inhabits, 

 but has not been actually recorded for, the intervening region or regions ; 

 and the marking above described may well be used to indicate this 

 probability, until actual information is obtained. 



{li). The area over which it is known that any species has become 

 extinct through natural causes within historic times, or has been exter- 

 minated by human agency, may be readily shown by covering the region 

 with parallel perpendicular lines, their colour of course indicating whether 

 the species in question is believed to have been a summer visitor, a winter 

 visitor, or a permanent resident in this region. On the other hand : — 



(j). The area over which it is known that any species has been 

 naturalized by human agency, more or less, within historic times may be 

 readily shown by covering the region with parallel horizontal lines, their 

 colour, again, of course indicating whether the species in question inhabits 

 the particular region as a summer visitor, a winter visitor, or a permanent 

 resident. In some cases it might be desirable to show the rate at which 

 this naturalization of a species has been extended.* This could easily be 

 done by means of fine lines, like isotherms of temperature, which might 

 each be dated, if desired. 



5. Generic Maps. — In addition to the maps indicating the 

 geographical distribution of species, it is desirable also to have 

 a series of maps showing the geographical distribution of 



* Such, for instance, as the spread of Passer domesticus over North 

 America, of Lejpus cuniculus over Australia, and (to select an example from 

 among the plants) of Elodea canadensis over Britain. 



