THE FACTORS OF EVOLUTION 197 



On this knowledge is based the system of artificial manuring 

 and cultivation of the soil, which through the exhaustive investi- 

 gations and experiments of the last two decades has been raised 

 to a science to which agriculture principally owes its increased 

 productivity. 



In order to obtain information concerning the nutritive value 

 and effects of a substance, water-cultures are employed, i.e., 

 the seeds are made to grow in vessels containing distilled water 

 to which the salts under examination are added in a chemically 

 pure form and in definite quantities. In a solution which 

 contains all the necessary salts land-plants flourish as well as 

 under natural conditions, but if we omit one or another of the 

 substances we observe immediately a retarding of the growth 

 and other changes. If, for instance, we omit to add iron the 

 young leaves become pale, assume a yellowish colour, and are 

 no longer able to decompose carbon dioxide. The plant is 

 therefore doomed to die, but so long as there are signs of life we 

 may again restore the green in the leaves by adding soluble 

 iron salts. Other substances are able to produce in the plants 

 entirely new characteristics. If, for instance, we grow maize 

 in a water-culture containing hypo-sulphuric magnesium the 

 plant produces flowers which differ so much from the normal 

 that we are unable to regard them any longer as maize-flowers. 



Chemical stimuli may greatly affect the life and development 

 of animals. For obvious reasons such experiments are mostly 

 conducted with aquatic animals. Several years ago Loeb 

 discovered the interesting fact that slight traces of potash when 

 added to the water accelerate development, while the adding of 

 a small quantity of an acid has a retarding effect. If we transfer, 

 during their developmental stage, the fertile eggs of echinoderms 

 into sea-water completely free from lime, the embryo-cells 

 continue to live, but they gradually fall asunder, and in place of 

 an organism we see ultimately a chaotic heap of individual cells. 

 If we decrease the amount of lime in the sea- water by only one- 

 tenth the process is likely to produce remarkable disturbances in 

 many animals. The Pluteus-larvae of the sea-urchin are normally 

 distinguished by the possession of long tentacle-like processes 

 which are supported by a special lime-skeleton, but in water with 

 a diminished amount of lime the formation of a lime- skeleton 



