io8 THE ORIGIN AND EVOLUTION OF LIFE 



ment of different parts of plants in individual development 

 and in evolution, plants appear to agree very closely with 

 animals. In both we observe that some characters evolve more 

 rapidly or more slowly than others in geologic time; also that 

 some characters develop more rapidly or slowly than others in 

 the course of individual growth. This may be termed charac- 

 ter motion or character velocity. 



This law of changes in character velocity, both in individ- 

 ual development (ontogeny) and in racial evolution (phylog- 

 eny), is one of the most mysterious and difficult to understand 

 in the whole order of biologic phenomena. One character is 

 hurried forward so that it appears in earlier and earlier stages 

 of individual development (Hyatt's law of acceleration), while 

 another is held back so that it appears in later and later 

 stages (Hyatt's law of retardation). Osborn has also pointed 

 out that corresponding characters have different velocities in 

 different lines of descent — a character may evolve very rapidly 

 in one line and very slowly in another. This is distinctively a 

 heredity-chromatin phenomenon, although visible in protoplas- 

 mic form. Among plants it is illustrated by the recent obser- 

 vations of Coulter on the relative time of appearance of the 

 archegonia in the two great groups of gymnosperms (/. e., 

 naked-seeded plants), the Cycads (sago-palms, etc.) and the 

 Conifers (pines, spruces, etc.), as follows: In the Cycads, which 

 are confined to warmer climates, the belated appearance of the 

 archegonium persists; in the Conifers, in adaptation to colder 

 climates and the shortened reproductive season, the appearance 

 of the archegonium is thrust forward into the early embryonic 

 stages. Finally, in the flowering plants (Angiosperms) with 

 their brief reproductive season, the forward movement of the 

 archegonium continues until the third cellular stage of the em- 

 bryo is reached. This is but one illustration among hundreds 



