( cxxii ) 



Mainly it would appear, from the operation of the destructive 

 forces above referred to, the number of individuals of most 

 species — though but for these forces the possible increase in 

 numbers is so immense — remains, taking one year with 

 another, substantially the same. 



Maintenance of existing " balance." 



The theoretical explanation is that the world is fully stocked, 

 that all places are filled up, so that the competition of species 

 prevents any permanent increase in the numbers of any one of 

 them, except at the expense of some other or others, all the 

 forces being thus balanced. This is a conclusion which seems 

 almost to follow logically from the known and admitted facts, 

 but the '' how," the process in detail, by which the normal 

 number * — i. e. on the average of years — whether of a common 

 or a rare species, is maintained, the various agencies by which 

 it is brought about, are admittedly difficult to realise. Darwin 

 tells us that the checks on increase and the relations between 

 organic beings which have to struggle together are recognised 

 to be extraordinarily complex. ** Battle within battle must be 

 continually recurring with varying success ; and yet in the 

 long run these forces are so nicely balanced that the face of 

 nature remains for long periods of time uniform, though 

 assuredly the merest trifle would give victory to one organic 

 being over another."! 



The forces whose interaction results in this delicate balance 

 and the methods of the interaction are so complex that proof 

 from observation must always be defective, except in some 



**• Weismaun, vol. i, pp. 46-95. 



t "Origin of Species, "pp. 87, 89, 133. Though a balance is maintained 

 it is not the balance, as the organisms between which it subsists are in a 

 state of change, generally a slow one, but at times rapid, sometimes from 

 causes which can be explained, such as human agency, as in the case of 

 the introduction of rabbits into Australia or sparrows into New York. 

 The causes are recognised to be generally obscure, and are considered 

 by some to be chiefly internal. An endeavour, attended with some 

 success, has been made to ascertain the causes of the rapid spread of 

 melanism in parts of England and North Western Germany ; the results 

 are summarised by Mr. L. Doncaster in the " Entomologist's Record" for 

 1906, pp. 165-8, 206-8, 222-6, 248-54. The very diverse surrounding 

 conditions under which melanism spreads (or fails to spread) show that 

 there is much to learn before its causes can be treated as ascertained. See 

 farther on this subject ]\Ir. Porritt's paper, read before the British Associ- 

 ation for the Advancement of Science in 1906. 



