Variation and Heredity 293 



be too much prejudiced by his results with the evening prim- 

 rose. If, as he supposes, many forms, generally ranked as 

 varieties, are really elementary species, it seems more proba- 

 ble that the mutation of a form may often be limited to the 

 production of one or of only a very few new forms. The 

 single variations, or sports, point even more strongly in favor 

 of this interpretation. Moreover, the general problem of 

 evolution from a purely theoretical point of view is very 

 much simplified, if we assume that the kinds of mutating 

 forms may often be very limited, and that mutations may 

 often continue to occur in a direct line. On this last point,! 

 De Vries argues that the evidence from paleontology cannot 

 be trusted, for all that we can conclude from fossil remains 

 is that certain mutations have dominated, and have been suffi- 

 ciently abundant to leave a record. In other words, the con- 

 ditions may have been such that only certain forms could 

 find a foothold. 



De Vries asks whether there are for each species periods 

 of mutation when many and great changes take place, and 

 periods when relatively little change occurs. The evidence 

 upon which to form an opinion is scanty, but De Vries is 

 inclined to think that such periods do occur. It is at least 

 certain from our experience that there are long periods when 

 we do not see new forms arising, while at other times, 

 although we know very few of them, epidemics of change may 

 take place. The mutative period which De Vries found in 

 the evening primrose is the best-known example of such a 

 period of active mutation. Equally important for the descent 

 theory is the idea that the same mutation may appear time 

 after time. There is good evidence to show that this really 

 occurs, and in consequence the chances for the perpetuation of 

 such a form are greatly increased. Delbceuf, who advocated 

 this idea of the repeated reappearance of a new form, has 

 also attempted to show that if this occurs the new form may 

 become established without selection of any kind taking place, 



