BOTANY 



compared with animals, is diminished. By means of the green 

 colouring matter, plants have the power of producing their own 

 nutritive substances from certain constituents of the air and water, 

 and from the salts contained in the soil, and are thus able to exist 

 independently ; while animals are dependent, directly or indirectly, 

 for their nourishment, and so for their very existence, on plants. 

 Almost all the other differences which distinguish plants from animals 

 may be traced to the structure of plants, or to the manner in which 

 they obtain their food. Another characteristic of plants is the un- 

 limited duration of their ontogenetic development, which is continuous, 

 at the growing points during their whole life. That none of these 

 criteria are alone sufficient for distinguishing plants from animals is 

 evident from the fact that all the Fungi are devoid of green pigment, 

 and, like animals are dependent on substances produced by green 

 plants for their nourishment. On the borderland of the two king- 

 doms, where all other distinctions are wanting, phylogenetic re- 

 semblances, according as they may indicate a probable relationship 

 with plants or animals, serve as a guide in determining the position of 



an organism. 



While it is thus impossible to distinguish sharply the two great 

 groups of living organisms from one another, a distinction between 

 them and lifeless bodies is readily recognised. Living organisms are 

 endowed Avith the quality of irritability, in which all lifeless bodies 

 are deficient. External or internal stimuli influence living organisms 

 to an activity, which is manifested in accordance with the require- 

 ments and conditions of their internal structure. Even in the 

 smallest and simplest known organisms the manifestations of life are 

 occasioned by a similar sensitiveness to external or internal stimuli. 

 It is, therefore, probable that the lowest living beings must have 

 possessed essentiall}'^ simpler properties than any organisms now 

 known, which would enable lis to connect them with non-living 

 substances. The substance Avhich serves as a basis for all develop- 

 ment must be supposed to have had an inorganic origin. So far as is 

 actually known, however, all living organisms have arisen only from 

 similar organisms. So far as experience has shown, spontaneous 

 generation is unknown. In the olden times it was a common sup- 

 position, which Aristotle himself held, that even highly organised 

 animals and plants could originate from sand and mud. In the 

 same degree that knowledge of the actual development of living 

 organisms was extended, the previously accepted cases of spontaneous 

 generation became more and more restricted, and were finally limited 

 to intestinal worms which could not otherwise, it Avas thought, be 

 accounted for, and to microscopic organisms, the origin of which also 

 was not understood. Now, for such organisms the possibility of a 

 spontaneous generation has been disproved by more modern investiga- 

 tions ; the history of the development of intestinal Avorms is knoAvn, 



