390 Mr. A. H. Cooke on the 



seems to think that a o^eograpliical variety can only exist " at 

 a great distance from the central point of the creation " of the 

 type. Why ? What is " a great distance " ? One hundred 

 kilometres, we are told, is too short a distance to constitute 

 the existence of a geographical variety. Would two hundred 

 kilometres be enough ? Would a thousand ? One is tempted 

 to ask, How many kilometres make a geographical variety? 

 Area lactea occurs in the Philippines in a form precisely 

 identical with that found at Suez. Are Philippine specimens 

 to be called a " geographical variety " because they live a 

 long way off the jMediterranean, but Red-Sea specimens an 

 " equivalent variety " because they live nearly within 100 

 kilometres of it ? 



Nor is it easy to see the force of the adjective in the term 

 under consideration. If " geographical " is meant to denote 

 that the intervention of distance between one geographical 

 point and another constitutes or compels variation, it states 

 what is not true ; if, on the other hand, that the type exists 

 at one place and the variety at another, it is merely an 

 " epitlieton otiosum " and may be dispensed with altogether. 



Again, the argument about the " central point of creation" 

 is surely rather confused. Issel says (1) that geographical 

 varieties originate at a great distance from the central point 

 of creation, and (2) that the Red-Sea " equivalents " cannot 

 be geographical varieties because they exist within 100 kilo- 

 metres of the Mediterranean. It looks as if he placed the 

 " central point of creation " for his four Mediterranean species, 

 which have " equivalent varieties " in the Red Sea, at Port 

 Said ! And if it is replied that the central point of creation 

 of a Mediterranean species cannot be established, but may be 

 assumed to be any point where the type occurs, the answer is 

 that in that case " central point of creation" is only a synonym 

 for " area of distribution." 



The truth is, distance quh distance has nothing to do with 

 the production or non-production of a variety. One can easily 

 imagine a stretch of sea-bottom two thousand miles in length, 

 and the same species of MoUusca existing, without the slightest 

 variation, at one end and at the other. What prevents 

 variation is not shortness of distance, but similarity of envi- 

 ronment ; and in the case of some of the MoUusca it is not 

 even necessary that the area of similar physical conditions 

 shall be unbroken. In the case of a species which, in a 

 certain stage of its existence, is free-swimming, the typical 

 form is possessed of means of locomotion far surpassing those 

 of the more sedentary species. 



This is no doubt the exjjlanation, though it has not yet 



