Maech 6, 1896.] 



SCIENCE. 



363 



ences ; it seems, therefore, that the topographical 

 factors in the distribution of terrestrial animals are 

 of more importance than climatic factors. 



The matter is different in the case of marine 

 animals. The oscillations of temperature are 

 not so extensive as those of the continents, and 

 the amount of these oscillations is very different 

 in different latitudes. Ortmann arrives at the 

 following conclusion from the given data: In 

 the equatorial regions of the oceans a nearly uniform 

 temperature prevails with only limited oscillations; 

 these oscillations increase with the latitude, reach 

 their maximum in the temperate and decrease again 

 to a smaller amount in the highest latitudes, the po- 

 lar regions. 



The conditions on the surface of the oceans 

 are of course different from those at some depth, 

 and the coasts have also an influence on the 

 litoral region. The oscillations of temperature 

 in the sea will appear especially in the upper 

 layers which are exposed to the direct inflvience 

 of the sun, that is principally in the litoral and 

 pelagial. With increasing depth they decrease 

 and are reduced to a minimum in the deep sea. 

 As is well known, the abyssal has a very constant 

 low temperature, and therefore we cannot ex- 

 pect any climatic effect on the distribution of 

 its animals. 



Effects of the topographical conditions on the dis- 

 tribution of animals. Combination of the climatic 

 and topographic principle. 



The continental is composed of a number of 

 completely separated landmasses; which ap- 

 proach each other closely in the northern hemi- 

 sphere, but which are always separated by the 

 sea. This character is fundamental and con- 

 ditions a different development of animals in 

 the separate land areas. These topographical 

 conditions are more important than the climatic 

 differences. A similarly extensive topographi- 

 cal segregration is seen in the fluvial; it is even 

 still more highly differentiated, consisting of a 

 very great number of topographically isolated 

 portions, which very often may be connected 

 with one another. These two life zones are 

 distinguished from the marine zones, which 

 are characterized by a more or less complete 

 continuity. The least continuity is shown in 

 the litoral, but even this zone is in uninterupted 

 connection along the coasts of the continents. 



In the abyssal and pelagial zones the continuity 

 is complete. 



We can, however, distinguish regions topo- 

 graphically separated in the litoral and pe- 

 lagial, but this is only possible by the combi- 

 nation of the climatic and topographical con- 

 ditions. In high northern latitudes the conti- 

 nents approach each other very closely ; in the 

 southern hemisphere they are removed from 

 each other. In the northern hemisphere, 

 where the continents are close together, the 

 litoral is continuous ; in the southern hemi- 

 sphere the pelagial has its broadest connections 

 around the southern end of the continents; in 

 both these regions totally different conditions of 

 temperature exist from those of the tropical 

 regions. They form therefore two completely 

 isolated regions, separated by the tropical por- 

 tions of the litoral. These are formed by four 

 large divisions, one on each side of the great 

 land areas, the old and new world. The pe- 

 lagial is only divided into two portions, the 

 Atlantic and Indo-Pacific, separated topo- 

 graphically by the two large landmasses. It is 

 a very important point that this topographical 

 separation of the tropical parts of these two 

 life regions is only made possible "by the cli- 

 matic differentiation of their circumpolar parts. 



The abyssal is not affected by such climatic 

 differences and cannot be separated topograph- 

 ically. 



Effect of the biological (biocoenotic) conditions on 

 the distribution of animals. Since migration 

 takes place in all directions the result will be 

 a conflict between the different immigrants. 

 Since in the resulting competition some forms 

 supplant others, we may use the expression 

 that the latter are checked by biological ob- 

 stacles. Especially in those cases the struggle 

 for existence is seen in its clearest form. 



The chapter concludes with some remarks on 

 the means of distribution of animals. Active and 

 passive means are distinguished. 



The means of distribution are different in the 

 different groups of animals; therefore, these 

 groups must show differences in their actual 

 distribution. Many animals have different 

 means of distribution in different stages of their 

 life history. Each single group must be treated 

 by itself for the determination of its distribu- 



