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, 
