THE NUMBERS OF ANIMALS 117 



That is the first point — the vast numbers of animals almost 

 everywhere. 



We have further seen that all animals possess an extremely 

 high power of increase, which if unchecked leads to over- 

 increase on a large scale, so that a " plague " of one species or 

 another is produced ; the best-known cases being mice and 

 locusts. All animals are exerting a steady upward pressure in 

 numbers, tending to increase, and they sometimes actually 

 do so for a short time. That is the second point. We next 

 considered the question of the desirable density of numbers 

 for a species, and we saw that each species tends to approach a 

 certain optimum density, neither too low nor too high, which 

 is not the same at different times or in different places. If 

 there are too few individuals the species is in danger of being 

 wiped out by unusually bad disasters, and if the numbers are 

 too great other dangers arise, the most important of which is 

 the over-eating of the food-supply. The latter is always the 

 ultimate check on numbers, but in practice other factors usually 

 come in before that condition is reached. 



25. We now have to consider the regulation of numbers, 

 the ways in which this desirable density of numbers is main- 

 tained. How do animals regulate their numbers so as to 

 avoid over-increase on the one hand and extinction on the 

 other ? The manner in which animals are organised into 

 communities with food-cycles and food-chains to some extent 

 answers the question. As a result of the existence of pro- 

 gressive food-chains, all species except those at the end of a 

 chain are preyed upon by some other animals. Snails are 

 eaten by thrushes, the thrushes by hawks ; fish are eaten by 

 seals, seals by sea leopards, and sea leopards by killer whales ; 

 and so on through the whole of nature. Most species usually 

 have a number of carnivorous enemies, but in some speciaHsed 

 cases may have only one. The latter condition is, however, 

 extremely rare ; it is the commonest thing in the world to 

 find a species preying exclusively upon another, but unusual 

 for a species to have only one enemy. Every species has also 

 a set of parasites living in or on it, which are often capable of 

 becoming dangerous when they are very numerous. So, in a 



