MUTATION 269 



is an advantage, the taller race has a better chance than 

 the smaller one. This statement does not exclude the 

 possibility that a short race might happen to beat out 

 in height a taller race, for it might more often mutate; 

 but chance favors the tall. In this sense evolution is more 

 likely to take place along lines already followed, if further 

 advantage is to be found in that direction. 



A rolling snowball that already weighs 10 pounds is 

 more likely to reach 15 pounds than is another that has 

 just begun to roll. The chance that a monkey could change 

 into a man is far greater than that an amoeba could 

 make the transition. The monkey has accumulated, so 

 to speak, so many of the things that go to make up a 

 man that his chance of reaching that goal is vastly greater 

 than the amoeba's. 



There is also a peculiarity of animals and plants that 

 assists greatly towards progress along lines already 

 started. The individual multiplies itself, and a new 

 mutant character that is advantageous becomes estab- 

 lished in a large number of individuals, or even in all indi- 

 viduals of the race. The number of individuals increases 

 the chance of a new random mutation along the path 

 already taken. It is true that the chance of a random 

 variation in the opposite direction is equally great, but 

 as this, by hypothesis, is the less advantageous direction 

 it will fail to establish itself in numbers. 



Darwin built up his evidence for natural selection and 

 even for evolution, on the artificial selection of variations 

 of animals and plants under domestication. It is in this 

 field that the student of Mendelism revels. Almost without 

 exception he finds that the domestic races of animals and 

 plants are built up by mutational differences. It is this 

 evidence that to-day is a hundredfold stronger for the 

 theory of evolution than it was in Darwin's time. 



The slightest familiarity with wild species will suffice 

 to convince any one that they differ from each other 

 generally, not by a single Mendelian difference, but by 



