THE EGGS AND YOUNG TADPOLES 3 



regarded as having its own special requirements, best met by spawning 

 at some particular season. Wc do not yet understand what these 

 special requirements are in each case but we may be sure that the forces 

 of natural selection are at work, and in the case of the frog, something 

 exists that determines the fact that, for this animal, early spawnini^ is the 

 right tiling. In a later chapter, it will be possible to show souie of the 

 advantages of being early. We can thus put the question as if it were a 

 problem that the frog has to solve: given the necessity for early 

 spawning, how can the dangers of death from cold best be com- 

 bated? 



If a committee of engineers and physical chemists were to sit round a 

 table to discuss the best material from which to make a heat-storage 

 device, they would give much attention to water. No other material 

 known to science has a greater specific heat, so that for any given 

 quantity of heat added or subtracted, the temperature changes are a 

 minimum. If, as is often the case, the object is to have a store of heat 

 accumulated in one period, to be given out in another, then convection 

 currents in the water are not a disadvantage, for both processes, heating 

 and coohng, are equally desirable. If, however, the only object is to 

 store heat, then the loss of heat in the cold parts of the cycle is a dis- 

 advantage. If only the water could be made solid, and if only the heat 

 could be generated within the mass, the ideal material would be at 

 hand. By producing the jelly, the frog has translated both these "ifs" 

 into reahty, with an efficiency not exceeded by any known material. 



The jelly of the frog's egg has often attracted the attention of zoolo- 

 gists and chemists, each from their own particular point of view. The 

 full story of this remarkable substance and its transformations can only 

 be written by taking account of both points of view simultaneously. 



The fully swollen jelly contains only about i per cent of dry matter. 

 This small quantity consists partly of salts from the water of the pond, 

 and the protein, wliich provides the structure, only amounts to about 

 0*3 per cent. This is all the material that the parent frog needs to pro- 

 vide in order to produce an envelope weighing about 300 times as 

 much, from the unlimited water of the pond, which provides the 

 remaining 99-7 per cent. The jelly surrounds the vitcllus, a small 

 spherical black body, answering the physical ideal for the absorption o( 

 radiant energy. (The word "egg" is sometimes used by authors to 

 refer to the black body alone, and sometimes to the complete object 

 including the envelopes, and the context must be examined to see wliich 



