Periodic Mutations 701 



fan itself, and secondly the combination of suc- 

 ceeding fans into a common genealogic tree. 



The composition of the fan as a whole in- 

 cludes more than is directly indicated by the 

 facts concerning the birth of new species. They 

 arise in considerable quantities, and each of 

 them in large numbers of individuals, either in 

 the same or in succeeding years. This multiple 

 origin must obviously have the effect of 

 strengthening the new types, and of heightening 

 their chances in the struggle for life. Arising 

 in a single specimen they would have little 

 chance of success, since in the field among thou- 

 sands of seeds perhaps one only survives and 

 attains complete development. Thousands or 

 at least hundreds of mutated seeds are thus re- 

 quired to produce one mutated individual, and 

 then, how small are its chances of surviving! 

 The mutations proceed in all directions, as I 

 have pointed out in a former lecture. Some are 

 useful, others might become so if the circum- 

 stances were accidentally changed in definite 

 directions, or if a migration from the original 

 locality might take place. Many others are 

 without any real worth, or even injurious. 

 Harmless or even slightly useless ones have 

 been seen to maintain themselves in the field 

 during the seventeen years of my research, as 

 proved by Oenothera laevifolia and Oeno- 



