BOTANY 



while animals are dependent 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, characterised by the firm walls of the simple organs, or to the 

 manner of obtaining food. Another characteristic of plants is the un- 

 limited duration of their ontogenetic development, which is continuous, 

 at certain points at least, 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 green plants for their nourish- 

 ment. On the borderland of the two kingdoms, where all other dis- 

 tinctions are wanting, phylogenetic resemblances, 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 difficult to sharply distinguish the two great groups 

 of living organisms from one another, a distinction between them and 

 lifeless bodies is readily recognised. Living organisms are endowed with 

 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 requirements and conditions 

 of their internal structure. Even in the smallest known organisms 

 all manifestations of life are occasioned by a similar sensitiveness to 

 external or internal stimuli. The question, however, continually arises 

 whether, in the smallest and simplest organisms at present discernible 

 with the highest magnifying power of the microscope, the ultimate 

 limit of possible life is actually represented. As this limit has always 

 been extended with the increased capabilities of optical instruments, it 

 would seem arbitrary to assert that it would now be impossible to 

 extend it still further. Nageli accordingly assumed that beyond 

 what is now made visible by the microscope there exists a world of 

 still more and more simple organisms. These he conceived of as 

 showing such a degradation of the vital processes that they finally 

 resemble mere albuminous bodies, which, he supposed, under certain 

 conditions might be produced by purely synthetic processes. In order 

 that a living organism may develop out of such albuminous bodies 

 it must originally have inherent in it the capability of development, 

 that is, the capability of variation and the ability to retain the 

 results of this variability as new qualifications. It must, in addi- 

 tion, have the capability of growth, or of enlarging the mass of its 

 body at the cost of foreign substances, and finally, the power of 

 reproduction, that is, of multiplication by a separation into distinct 

 parts. 



For the substance itself which serves as a basis for all development, 

 the supposition of an inorganic origin would not be incredible ; it 

 would even be possible to imagine that, under certain conditions, 

 this substance is continually in process of formation. On the other 



