COMPARATIVE RAPIDITY OF EVOLUTION IN 

 VARIOUS PLANT TYPES 



PROFESSOR EDMUND W.'SINNOTT 

 Connecticut Agricultural, College 



During the course of evolutionary development among 

 the higher plants certain groups have evidently altered 

 with such exceeding slowness as to retain their ancient 

 constitution practically intact for very long periods; 

 whereas others, by their more rapid accumulation of 

 heritable variations, have during the same time under- 

 gone far-reaching changes and become developed into 

 new and radically dissimilar types. These differences in 

 the rate of evolution are apparently due in part to dif- 

 ferences in mutability, in the extent to which hybridiza- 

 tion has occurred, in the degree of diversity presented by 

 the environment or in the keenness of the struggle which 

 is waged for survival. The purpose of the present paper 

 is to call attention to still another factor which seems to 

 be of much importance in determining the rapidity of 

 evolutionary change among plants, namely the length of 

 the generation or period from seed to seed. 



The time necessary for the attainment of reproductive 

 maturity among plants is definitely correlated with the 

 growth-type to which a species conforms. Trees are very 

 slow to mature, some arriving at an age of eighty years 

 or more before their first flowering time and very few in- 

 deed, under natural conditions, fruiting before the tenth 

 season. The scanty available observations seem to indi- 

 cate that the average period from seed to seed in arbores- 

 cent forms is in the vicinity of twenty years. Among 

 shrubs the generations are decidedly shorter, varying 

 usually from three to ten years. The herb is the most 

 rapidly maturing of all, a single year sufficing to develop 

 seed in an annual and but two (rarely more) in a biennial 

 or perennial. Most herbaceous species will thus have 



