364 THE FRESH-WATER LOCHS OF SCOTLAND 
• 
lower Crustacea), in spite of a temperature little above, and sometimes 
definitely below, 0° C. 
However, it is particularly in the case of fertilised ova that the 
power of resisting extreme cold is most marked, for in several groups 
specially protected winter eggs are produced, which appear able to 
survive almost any degree of cold. There are the gemmules of the 
Spongillidae, and the hard-coated winter eggs of certain Turbellaria 
and Rotifers ; also the resistant eggs of a number of Entomostraca 
(including the ephippial eggs of the Cladocera), and the so-called 
statoblasts of fresh-water Polyzoa. These are produced by the parent 
on the approach of cold weather, and in the spring give rise to new 
individuals, to replace the adults which have perished. 
Some observations of Brauer confirm our belief that the winter 
eggs are produced with the definite object of resisting cold, and at the 
same time afford an interesting example of how inherited characters 
may continue to exert their influence under altered conditions. He 
found that the eggs of a certain species of BrancMpus would not 
develop at all, until after they had been reduced to the temperature 
of meltino; ice. 
Complete desiccation is a condition which is fatal to all organic life, 
so that those forms which are able to survive the drying up of a pond 
or stream have acquired some means of retaining a sufficient amount 
of moisture to make their continued existence possible. As this is 
obviously an unfavourable condition, it is not surprising that organisms 
lead during it a latent life which is strikingly comparable to that 
induced by extremes of cold. The African mud-fish {Protopterus) 
buries itself in the mud, and secretes an impervious cocoon in which 
it can exist for months, until the coming of the rainy season. Some 
adult Rotifers are capable of encysting themselves, and, in this state, 
of surviving long periods of drought, and the same is true of immature 
specimens of a species of Cyclops^ and of certain Protozoa {Amceha and 
Infusoria). 
It is nevertheless but a small assemblage of forms in which the 
adult is able to resist desiccation, compared with the much larger 
assemblage in which the power of resistance is confined to the repro- 
ductive bodies. This is precisely what we have seen to be the case as 
regards resistance to extreme cold ; indeed, the two phenomena are 
closely akin, and it is not perhaps surprising that a protective coating 
to the fertilised ovum suitable for the one purpose should afford 
adequate protection in the other. 
In the vegetable kingdom, many Fungi and Algae produce highly 
resistant spores which serve for the perpetuation of the species, and 
the seeds of the higher aquatic plants can survive a dryness which 
would kill the parent stock. Among animals, resistant reproductive 
