THE ANIMAL COMMUNITY 6i 



able to dislodge those of moderate size, not usually more than 

 45 millimetres across. 



13. These are three rather curious cases of what is a 

 universal phenomenon. Man is the only animal which can 

 deal with almost any size of food, and even he has only 

 been able to do this during the later part of his history. 

 It appears that the very early ancestors of man must have 

 eaten food of a very limited range of size — such things as 

 shellfish, fruits, mushrooms, and small mammals. Later on, 

 man developed the art of hunting and trapping large animals, 

 and he was thus able to increase the size of his food in the 

 upward direction, and this opened up possibilities of obtaining 

 food in greater bulk and variety. After the hunting stage 

 came the agricultural stage, and this consisted essentially in 

 the further development of the use of large animals, now in a 

 domesticated state, and in the invention of means of dealing 

 with foods much smaller than had previously been possible, 

 by obtaining great quantities of small seeds in a short time. 

 All other animals except man have their food strictly confined 

 within rather narrow limits of size. The whale-bone whale 

 can feed on tiny Crustacea not a thousandth of its bulk, while 

 the killer whale can destroy enormous cuttle-fish ; but it is 

 only man who has the power of eating small, large, and medium- 

 sized foods indiscriminately. This is one of the most im- 

 portant ways in which man has obtained control over his 

 surroundings, and it is pretty clear that if other animals had 

 the same power, there would not be anything like the same 

 variety and specialisation that there is among them, since the 

 elaborate and complex arrangements of the food-cycles of 

 animal communities would automatically disappear. For the 

 very existence of food- chains is due mainly to the fact that any 

 one animal can only live on food of a certain size. Each stage 

 in an ordinary food-chain has the effect of making a smaller 

 food into a larger one, and so making it available to a larger 

 animal. But since there are upper and low^er limits to the 

 size of animals, a progressive food-chain cannot contain more 

 than a certain number of links, and usually has less than five. 



14. There is another reason why food-chains stop at a 



