170 THE president's ADDRESS. 



any precision, and until we have a more thorough knowledge 

 of the range of forms to be dealt with, it is premature to try 

 to work out the lines of evolution and descent of the Protista 

 as a whole, a study indispensable if the classification is to have 

 any approach to finality. Nevertheless, I believe it is possible 

 to lay down certain guiding principles which indicate the general 

 trend and direction of evolution in the simplest forms of life, or 

 at all events of that main stem which leads on towards the 

 higher organisms ; bearing always in mind that other branches 

 may have taken other directions leading sideways or even down- 

 wards rather than upwards, just as in the Metazoa, one of the 

 lowest branches, that, namely, represented by the Sponges, 

 clearly does not lead on to any higher form of life. 



But first I must say a few words about certain distinctions 

 commonly drawn among living organisms. In the case of the 

 visible world of living beings familiar to us in every-day life, 

 the most obvious and natural division is into animals and plants. 

 No distinction could be sharper or more incontestable. Conse- 

 quently the human mind tends almost instinctively to place 

 any living thing under one or other of these categories, and 

 can scarcely be brought to conceive of any form of life as being 

 possibly neither animal nor plant. 



When we inquire on what the distinction of animal and 

 vegetable is based, it is seen at once that it depends upon the 

 manner in which the organism builds up its living substance, 

 that is to say, upon those vital processes which the physiologist 

 sums up under the term metabolism. All living bodies contain 

 substances of great complexity termed proteids, the most complex 

 chemical substances known to 'exist. A plant has the power of 

 building up its proteids from simpler chemical substances ; an 

 animal has no such power, but must be supplied with food 

 containing ready-made proteids, hence it cannot exist without 

 plants or other animals to feed upon. In plants, however, the 

 metabolism is by no means of one unvarying type, since green 

 plants, by the aid of their green pigment, chlorophyll, or colour- 

 ing matters of allied nature, can utilise the energy of the sun's 

 rays in order to build up their proteids from the simplest 

 inorganic salts and gases, while saprophytes, such as fungi, which 

 have no chlorophyll, require for their nourishment organic sub- 

 stances that is to say, substances produced by the vital activity 



