NATURE AND OEIGIN OF FRESH-WATER ORGANISMS 363 
the fact that they float at the mercy of every current ; while among 
the groups which are poorly or never represented in fresh water we 
find a large proportion of forms which pass through a free-swimming 
larval stage. That this factor has been of great importance is con- 
firmed when we examine those organisms which have effected a 
conquest of fresh water, for we find that in the majority of cases a 
free-swimming stage during development has been suppressed. 
Of almost equal significance are the temperature differences 
between the waters of the ocean and of inland areas. It is quite evident 
that comparatively small masses of water, such as even the largest 
rivers and lakes, are more liable to variations of temperature than the 
vast waters of the ocean. In the tropics, a comparison between the 
ocean and a really large lake may show differences of little importance ; 
but on the other hand, where the mass of water is small, as in ponds 
and streams, the contrast becomes very marked, and there is the 
additional danger that the water may entirely dry up. In temperate 
and colder climates there ai-e often greater extremes, and in many 
cases equal danger to life, on account of the freezing of the water. 
The inhabitants of the more uniformly warm ocean, which is never 
subject to drying up or to freezing, will certainly find a difficulty in 
colonising where there are these undesirable features, and in fact it is 
only the forms which can fully adapt themselves to such altered circum- 
stances that can ina^ke the change. 
While these conditions have probably checked migration in a 
number of instances, there are types belonging to several groups 
which have become able to withstand high or low temperatures, 
as the case may be, or have devised means of surviving desiccation 
and freezing. 
A few examples will serve to show the extremes which can be reached 
by forms which have been successful colonists. Certain Algae and 
Bacteria have been found living in the water of geysers at temperatures 
up to 80° C, and a fish {Haplochrornis desfontaincsi) lives in Tunis in 
hot springs with a temperature of 75° C. 
On the other hand, it is well known that most of our familiar 
plants are not killed by frost, though their vital activities are sus- 
pended, and a temperature of a little over 0° C. is sufficient for vigor- 
ous growth in the case of our earliest spring flowers and the plants of 
alpine and polar regions. There are animals, too, which can survive a 
temperature below freezing-point, but the cold in many cases induces 
a complete cessation of the ordinary functions of life. Frogs and 
toads, many fishes, and certain Mollusca can undoubtedly withstand 
such cold and resume their normal existence on the necessary increase 
in warmth. Further, it is a fact that the seas in the Arctic and 
Antarctic regions are often well stocked with life (largely Algae and the 
