No. 509] THE CATEGORIES OF VARIATION 269 



To the extent that the organism is a whole of interde- 

 pendent parts, to just that extent it gives evidence of not 

 being a piece of mosaic work and hence removes the neces- 

 sity for a hypothesis of discrete germinal units. De 

 Vries even goes so far as to say that "in order to be 

 correlated the characters must begin by being independ- 

 ent entities which through some later means may come 

 into relation with others. ' ' At one time it is argued that 

 the existence of pangens is proved by the fact that the 

 parts of the organism are independently variable; and 

 now it is said that we must assume that pangens must 

 exist to account for the parts being correlated; that is, 

 for the fact that they are not independently variable ! 



The most salient feature of the mutation theory is that 

 the process of evolution is conceived to take place by 

 sudden steps of considerable magnitude. This has been 

 heralded with eclat as enabling us to get rid of certain 

 difficulties inherent in the Darwinian theory, such as the 

 assumed absence of intermediate forms between existing 

 and fossil species and even the contention that the geo- 

 logical history does not afford time enough for the process 

 of evolution as it was formerly conceived to take place. 

 But it is not correct to say that a mutation is neces- 

 sarily a large variation. Mutations may be very small 

 steps, falling far within the limits of ordinary fluctuating 

 variability, as is especially emphasized by De Vries in 

 his later writings. 



In groups (such as brambles, roses, buttercups, willows and many 

 others) where large numbers of species are closely allied, the differ- 

 ences between any two of them become smaller, and the number of 

 distinct forms increasing, the distinction in the end may become re- 

 duced to a single differential mark for each two neighboring types. 

 Such differences must be assumed to be produced each by a single 

 mutation. 



In the light of experiments made at Svalof, De Vries 

 now concludes that "ordinary varieties of cereals are 

 built up of hundreds of elementary forms which with few 

 exceptions have hitherto escaped observation. The high 

 variability which is commonly attributed to our ordinary 



