152 LECTURES ON BIOLOGY 



figure of 1,600 trillion descendants of one bacillus. According 

 to Fischer, this mass of bacilli would, in spite of the extreme 

 minuteness of the individuals, yield in the dry state a weight 

 of 100,000 kilogrammes. It would therefore require a gigantic 

 experiment to let only one of these minute bacilli grow and 

 multiply during twenty-four hours to the fullest of its ability. 



But long experience has shown that the number of the 

 individuals of a species within a certain definite area maintains 

 itself during the course of great periods usually on the same level, 

 however great may be the temporary fluctuations. The supposi- 

 tion is, of course, that the conditions of life remain the same. 

 Devastating epidemics, famine, great floods, long periods of frost 

 or drought may, of course, reduce the species in a short time to 

 partial or actual extinction. On the other hand, we know from 

 numerous instances that animals as well as plants which are 

 suddenly introduced into new and favourable conditions multiply 

 often in a most extraordinary manner. No one imagined when 

 the first rabbits were brought from Europe to Australia what 

 a terrible scourge they would some day become to that country. 

 Threatened by no natural enemy, and finding food in super- 

 abundance, the animals increased at such enormous ratio that 

 they soon became a veritable pest, for the extermination of which 

 the Government in vain promises large premiums. 



Cattle and horses belong to the animals with a low ratio of 

 reproduction. If it had not been proved beyond doubt we should 

 not believe it possible that the enormous herds that to-day 

 inhabit the wide pampas of South America are the descendants 

 of European races. The vegetable kingdom offers many other 

 similar instances. The plains of La Plata are for miles covered 

 with artichokes and another large species of thistle, yet it is 

 not so long ago that the first plants were brought from Europe. 



As with the increase in the numbers of the species there is 

 observed a corresponding increase in the desire for reproduction, 

 it might be thought that multiplication can proceed without 

 limitation. That is, however, not so ; for we find that very soon 

 an average number is reached which constitutes the maximum 

 even for the changed favourable conditions ; increase beyond that 

 point is rigidly prohibited by the conditions of life. If now this 

 normal number of a species is not to be exceeded there must not 



