60 General Part. 



repair to the land for reproductive purposes. Reptiles have also 

 contribated to the animal life of the sea (Sea-snakes, Sea-turtles). 

 Amongst Birds there are none which have become entirely marine, 

 although many are connected more or less closely with the sea, of 

 those most so, the Penguin and many other Swimming Birds. Of 

 living Amphibia there are no entirely marine forms. Only a few 

 Spiders live in the sea, and hardly any Insects. Like the 

 terrestrial, the marine fauna is modified according to the very 

 different conditions existing in different parts of the ocean, the dis- 

 similar character of the bottom, the depth, etc. The littoral fauna is 

 different from that at greater depths, which again varies according 

 to the bottom. The salinity is also of the greatest importance, for 

 greater salinity is universally favourable to animal life ; it suits a 

 greater number of species, though less salt water may also be rich in 

 iiidividuals. 



This is very clearly illustrated by comparing the state of the salt Kattegat 

 ■with that of the less salt Western. Baltic, and the almost brackish East Baltic. In 

 the Kattegat there is a tolerably rich fauna, but already the- north end of the 

 Sound, where the salinity is less, contains a scantier collection. Most species 

 which live in the Kattegat are represented here, but for the most part by smaller 

 epecimens and in fewer numbers. Southwards in the Sound, as in the entire 

 western part of the Baltic (south of Denmark), very many of the Kattegat forms 

 bave disappeared ; others are indeed present, but dwarfed, or in the case of the 

 Molluscs, thin shelled. Lastly, only a fraction of the fauna of the West Baltic 

 is found in the fresher East Baltic, and this is also the case in that part of the 

 latter (sovith of Sweden), which, as regards climate, does not materially differ 

 from the West. 



Some fresh water animals (pike, perch) may also occui- on the coast in 

 water of slight salinity ; and on the other hand, some marine animals may be 

 present in fresh water, the flounder {Fleuronectes flesus). Also, some Eish go to 

 spawn either from fresh water into the sea (eels), or conversely (salmon, trout, 

 sturgeon). Sudden immersion in sea water is fatal to most fresh-water 

 animals, and most marine forms are similai-ly affected by fresh water. On the 

 other hand, many endure, to a certain extent, a gradual salting or freshening of 

 the water. 



Temperature has often a great influence upon the dispersal 

 of animals ; this is especially striking in the terrestial fauna 

 which, other conditions being the same, is much richer in hot than 

 in cold districts; in the very coldest, it is almost entirely absent, 

 or at least, reduced to a minimum. This is due not only to a higher 

 temperature being universally favourable to animals, but also to the 

 dependence of animals upon plant life, which is much affected by 

 temperature. Since the temperature of sea water does not sink as low as 

 that of the atmosphere, the sea in the coldest regions may harbour 

 a rich fauna, although it cannot compare with that of tropical seas. 



The more general features of the dispersal of animals have now 

 been touched upon; in what follows, some special adaptations will 

 be considered. 



