30 GEOGEAPHICAL DISTEIBUTIOlT. 



of self-locomotion, and, indeed, in the case of those pelagic forms 

 whose dispersion or "migi-ation" is less a matter of volition than 

 the result of an interaction of extraneous physical causes there 

 would seem to be no barriers set to a practically universal distribu- 

 tion. But here, too, Nature has set a limit to the possibilities of 

 migration, and, therefore, even among those lower forms which 

 might be considered best adapted for withstanding the varying 

 physical vicissitudes of their surroundings we meet with but very 

 few species whose distribution might be said to be in any way 

 cosmopolitan. The free-swimming pteropods, or winged -Mol- 

 lusca, and medusoids, although exhibiting individual examples 

 of very broad distribution, are still more or less restricted specifi- 

 cally to well-defined oceanic areas, whose boundaries may in a 

 measure be dependent upon the prevalent Surrounding water- 

 currents. Shells of the Spirula Peronii, a member of the two-gilled 

 order of cephalopods, are met with almost all over the oceanic bor- 

 ders, as well in the temperate as in the tropical zones, but, owing 

 to the extreme rarity of the animal itself, which has been observed, 

 perhaps, but a half-dozen times, it is impossible to say what the 

 exact, or even approximate, range of the species is, and, conse- 

 quently, of how much of the area of the distribution of the shell it 

 partakes. The common form of argonaut (Argonauta argo) is found 

 in the tropical parts of the Atlantic, Pacific, and Indian oceans, and 

 in the Mediterranean Sea, and it has been met with as far north 

 in the Atlantic as the New Jersey coast, and as far south as the Cape 

 of Good Hope. The animal might, therefore, be said to be almost 

 cosmopolitan. 



It may be laid down as a fundamental law in geographical dis- 

 tribution that the areas inhabited by a given species are continuous 

 with each other; in other words, we do not find, except at rare 

 intervals, and under peculiar circumstances, the same species of 

 animal inhabiting distantly-separated localities, in the intei-val be- 

 tween which no individual of the species is to be met with. Thus, 

 in the entire range of the leopard there occurs no district of any 

 significance where the animal may not be confidently looked for, 

 and which would negatively tend to render its distribution discon- 

 tinuous. And the same may be said of the hundred or more 

 degrees of latitude prowled over by the couguar, an animal whose 

 home is at one place the lowland forests, at another the elevated 



