100 LIFE IN THE SEA [PART I 



of the bottom-living invertebrata, but these have not been studied 

 to the same extent as have the plankton and the fishes. 



All throughout the year there is a more or less regular 

 change in meteorological conditions. The temperature of the air 

 is continually changing : violently at times, because of changes in 

 winds, as cyclone succeeds cyclone, or are interspersed with the 

 more welcome anticyclonic systems; but throughout the year 

 rising generally to its maximum and again falling to its minimum. 

 The temperature of the sea too undergoes much the same 

 variations, but these are more regular than in the case of the 

 atmosphere. Sometimes the difference between sea and air 

 temperatures is relatively great, sometimes relatively small. The 

 duration and intensity of sunlight vary from day to day ; and to 

 some extent at least there are variations in the salinity of the 

 water. All these changes in physical conditions react on the life 

 processes of marine organisms, but the primary cause is the 

 annual change in the temperature of the water. Just as there is 

 a great outburst of vegetation on the land in the spring and 

 summer, so there is a spawning season in the spring and early 

 summer in the sea, and in the case of most marine organisms this 

 habit of reproduction during these months has been stamped upon 

 them by heredity. We thus find that the hinge round which most 

 seasonal variations in the abundance of animals in the sea turn is 

 the annual reproductive phase sometime or other during the 

 first six months of the year. 



There is only food for a limited (though of course very great) 

 mass of life in the sea, and any change which is favourable to the 

 multiplication of the individuals of one species must necessarily 

 react on other organisms. The sudden production of the vast 

 numbers of larvae, which result from the spawning of hosts of fishes 

 and invertebrates in the spring, must lessen the numbers of 

 diatoms, protozoa, or micro- Crustacea on which those larvae feed. On 

 the other hand, the appearance of the eggs and larvae means 

 that food is provided for other predatory animals : thus haddock 

 follow the spawning herring and gorge themselves with the eggs 

 which the latter fishes deposit on their spawning grounds, and to 

 such an extent does this destruction take place that hundreds of 

 boxes of haddock have been landed by fishermen, and all these 



