SUFFICIENCY OF SMALL VARIATIONS 97 



children are not resistant. It would therefore seem that 

 adaptation to the environment is a very gradual process. 



The converse of this also holds good. We find that 

 all the available evidence goes to show that the change of 

 environment has always been very gradual. Formerly it 

 was believed that catastrophes occurred during various 

 geological periods. Now, however, geologists appear to be 

 agreed that there is no evidence suggesting that changes 

 took place more rapidly in former times than they are 

 taking place at present. 1 We are in fact obliged to believe 

 that a sudden change of environment can have rarely been 

 produced in relation to any organisms in the past, except by 

 some such artificial means as occurs in the case of trans- 

 porting an English dog to India. 



The argument brought forward most frequently against 

 small variations is, that they are insufficient to account for 

 the production of new species owing to their smallness, and 

 to the enormous amount of time that would be involved. 

 It is argued that these " chance variations " are very unlikely 

 to be in a favourable direction sufficiently often. Let us 

 consider for a moment what kind of chance we have to deal 

 with. We know that the number of individuals of any 

 given species remains about the same during very long 

 periods of time. When we do get a large increase in the 

 number of individuals, the reasons for the increase can 

 generally be accounted for more or less easily, and it is 

 due to some such causes as the removal of an enemy, or an 

 increase of food supply. These cases are, however, excep- 

 tional. As a rule, the number of individuals remains about 

 the same. We may take it that the number of offspring 

 surviving is generally about the same as the number of 

 parents that have produced all the offspring. Now, the 

 house-fly lays five or six batches of eggs with about 120 to 

 150 in each. In hot weather a generation occupies about 

 three weeks ; that is to say, that from the time the egg is 



1 Geikie, Sir Archibald, Presidential Address to the British Association, 1892 ; 

 Poulton, op. cit., pp. 1-45 ; and many other authorities. 



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