56 FROM DUST TO DUST. 



of available plant food to the higher forms of vegetable life, from 

 which the Aniijial World derives its sustenance. 



There is, however, really no boundary line between the vege- 

 table and animal kingdoms ; the distinction which Cuvier and 

 others made between them all break down under examination. 

 Some organisms, such as Mycetozoa, appear to belong, at different 

 stages of their existence, to both kingdoms. Broadly speaking, 

 however, plant life is nourished and sustained from without; whilst 

 animal life is supported by digestion of food from within. In 

 other words, plants have their roots outside, and animals have 

 theirs inside themselves ; except those lower organisms which 

 obtain their food from the fluid media in which they are bathed, 

 and others which lead an entozoic and parasitic existence, It 

 became necessary for those animals which were attaining a higher 

 complexity of organisation, and had to seek food by moving about 

 from place to place, to take their roots with them. This com- 

 menced by what is known as "gastrulation," passing through every 

 stage, from mere invagination of the ectoderm to the gastro-intes- 

 tinal system of the higher mammals. It also became necessary that 

 the higher animals should be, to a great extent, independent of 

 the immediate assistance of external ferments ; consequently they 

 have acquired the power of digesting and assimilating the organic 

 proteids, carbo-hydrates, and fats, by means of their own ferments, 

 such as Ptyalin, Pepsin, Trypsin, Amylopsin, and the Succus 

 Entericus. These are unorganised ferments, which were formerly 

 supposed to exert merely a chemical action. They, however, 

 possess the character of living things, except that they cannot be 

 identified as living organisms. 



also contained a small amount of other algce, had the power of fixing free 

 nitrogen to a large extent. The same author also describes a number of 

 experiments with heterogeneous mixtures of algae and bacteria, and shows how 

 in each case the capability of fixing free nitrogen is greatly increased by the 

 addition of dextrose to the nutritive substratum, from this, and also from the 

 fact that such mixtures of algae and bacteria, which are capable of fixing free 

 nitrogen when exposed to light, cannot be shown to assimilate it in the dark, 

 he concludes that although in no case has it been proved that algse by them- 

 selves possess the power of fixing free nitrogen, yet they are in a symbiotic 

 relationship with the nitrogen-fixing bacteria, and he regards it as probable that 

 these latter draw on the assimilation products of the algaj to supply the carbon 

 they require in growth." 



