Chapter XLV. 



Maintenance of the Plant Species. 



The methods by which individual plants are maintained as 

 units in Nature may be regarded as methods, also, for main- 

 tenance of the species. A species consists of all the individ- 

 uals, living and extinct, that are judged to form a single and 

 definite line of inheritance and are of practically identical 

 structure. Thus, one speaks of the human species, meaning 

 not only the men and women of to-day, but also their an- 

 cestors. In the same manner one may take into the mind 

 the conception of a plant-species, such as the dandelion, or 

 the white pine. While the lives of individuals are compara- 

 tively brief, the life of a species may be of long duration, 

 varying, perhaps, from a few hundred to many millions of 

 years. Most species are more than ten thousand years old — at 

 least this is the belief that has been strengthened by researches 

 in geology and in the history of development. 



In order to continue the chain of individuals that constitutes 

 a species, the living substance both among plants and animals 

 has the universal habit of separating portions, usually single 

 cells, from the mature body, and these, if properly situated, 

 will build up new bodies from the organic or inorganic world 

 by which they are surrounded. Such a process is termed re- 

 production. It is analogous to growth and may be regarded as 

 a function of living substance that has become specialized 

 owing to the establishment, for each kind of living thing, of a 

 life-period. The normal life-period may vary in different or- 

 ganisms from a few days to many hundreds of years. Some 

 individuals are comparatively ephemeral, while others are able 

 to maintain themselves over periods almost as long as that of 

 human history. Whatever is the duration of the life-period, it 

 may always be considered as expressing a ratio between the 

 bodily vigor of the organism and the average sum total of 



