Mutation Within Mutation Periods. 207 



ical and biological environment remains the same we 

 must suppose that the species will not change. But if 

 the plant extends its range, or if those with whom it com- 

 petes for the means of subsistence, change in any way, 

 the opportunity for the appearance of mutations is at 

 once given. Either of these occurrences might result in 

 a shorter or longer time in a rapid and considerable in- 

 crease in the number of individuals and this might be the 

 cause of the appearance of mutations on the scene. For 

 a rapid multiplication of this kind presupposes the germi- 

 nation of such seeds as under ordinary circumstances 

 either w^ould not have germinated at all, or would have 

 come to nought. This might be the case for example 

 with seeds of weak lateral branches, of the tips of in- 

 florescences or of flowers from accessory buds and so 

 forth. 



But these are after all only suggestions; and I feel 

 strongly that we ought to make this matter a subject 

 for experimental inquiry; to look for species which hap- 

 pen to be going through a period of mutation and still 

 more to discover what are the factors which will enable 

 us to induce such a period in a species at will. We have 

 a doctrine of descent resting on a morphological founda- 

 tion. The time has come to erect one on an experimental 

 basis. y/ 



§ 28. THE PHENOMENON OF MUTATION WITHIN THE 

 LIMITS OF THE MUTATION PERIODS. 



Observations on periods of mutation have not yet 

 been made. On the other hand many attempts based 

 on a priori considerations have been made to discover 

 what the phenomenon of mutation may be expected to 

 be like. 



