38 EVOLUTION AND GENETICS 



and incaj)able of competing with the original form. 



In this connection it is important to keep in mind 

 the fact that animals and plants are extremely com- 

 plex machines that are highly adapted to the condi- 

 tions of life in which they live. Any change, and 

 especially any great change in them, is far more 

 likely to throw them ont of balance with their envi- 

 ronment than to bring to them an advantage, but it 

 is possible, nevertheless, that some of the changes, 

 however slight, might be beneficial, especially those 

 that add to or diminish slightly some important char- 

 acter or function already present. These changes 

 might furnish materials for evolution. 



Darwin rested his case for evolution on the ob- 

 served small differences that all animals and plants 

 show. Now, while we realize today that many of these 

 slight differences are not inherited, we also know 

 that amongst them there are some that are inher- 

 ited, and that these, so far as we know, have arisen 

 as mutations. At present there is no evidence that 

 will stand the test of criticism in favor of any other 

 origin than that all known variations owe their ap- 

 pearance to the same process of jiiutation that also 

 produces the larger differences; and, I repeat, that 

 there is much explicit evidence to show that very small 

 differences, that add to or subtract from characters 

 already j)resent, do appear by mutation. 



Finally there is evidence that the differences 

 shown by individuals in nature are inherited in the 



