DISTRIBUTION AND ABUNDANCE 9I 



rather to rest on a belief, obtained from analogies, that systems that 

 tend to some mean value always arise in this way. Nicholson's views 

 are not universally accepted, and Solomon (1949) has brought together 

 in a wide review many of the other theories on this subject. It seems 

 curious that the very large number of cases in which complex systems 

 are maintained at levels fluctuating within limits, but which arc 

 demonstratively not due to tliis kind of control were not used as 

 analogies. If examples had been taken from complicated systems such 

 as are found in molecular physics, life assurance or the imiumerable 

 properties examined in industrial control laboratories, the belief that 

 there must always be a restoring force related to the displacement 

 could surely not have become so widespread. 



In a volume of gas at constant temperature, the velocity of the 

 molecules is maintained at a constant average merely because of a 

 statistical set of probabihties. There is notliing that tends to accelerate 

 a slow molecule merely because it is slow — all that happens is that in 

 this comphcated situation, there is a slightly greater chance that at the 

 next colhsion it will be accelerated rather than slowed. Except in this 

 sense, which is not that usually used in these discussions, there is no 

 * Velocity-dependent" control and, in fact, a slow molecule is some- 

 times slowed still further. 



If we now look to oiif own species for an ecological example of 

 control by purely statistical means, we may consider the population 

 density of Homo sapiens in one of its ecological niches, the 4 o'clock 

 week-day train from Manchester to London. The numbers of the 

 animal fluctuate between fairly small limits — the train is never empty, 

 but I have never seen anyone left behind. Not the shghtest attempt is 

 made by British Railways to control the numbers — anyone can buy 

 a ticket and board the train. The only reason why all Manchester 

 does not try to do so, or all try to stay at home, is that human life is so 

 complicated that the chance of either of these events is negligible. If 

 the train is crowded on Tuesday, it does not in the least follow that 

 it will be less crowded on Wednesday, although this is probable. On 

 Sundays, it is very different. The railways would then be used too 

 little, so British Railways issue cheap tickets, and so exert density 

 control. 



The death rate is not in the least influenced by the experience of life 

 offices, but, after an exceptional period of very many or very few 

 deaths, the mean value is soon regained. In some countries, there may 



