Evolution 



Now it is a somewhat remarkable fact that we do not know 

 whether or not all the humbler forms of life possess a nucleus. 

 It was formerly believed that a considerable number of one-celled 

 organisms were devoid of the bod}- in question, but in most of 

 such it has been shown that nuclear matter is present, though it 

 may be distributed, in small portions, throughout the cell. If 

 organisms do exist which consist of a cell without a nucleus, we 

 must regard them as the simplest of living things. In any case, 

 the formation of a nucleus, a process by which a kind of central 



government was formed, was 

 probably one of the great early 

 steps of Evolution. 



The life-history of an ordin- 

 ary one-celled organism may 

 be briefly summed up. It 

 absorbs nourishment and 

 energy, adds to its substance 

 until it reaches a certain fairly 

 definite size, and then divides in 

 two, the halves separating, and 

 going each its own way. In the 

 world of one-celled organisms 

 there is no ' death from natural 

 causes.' The individual is 

 potentially immortal, except in 

 so far as we may regard the individual life as ceasing when divi- 

 sion takes place. Death occurs only, as we say, accidentally 

 for example, from starvation or from the attacks of enemies. 

 A number of simple unicellular organisms are shown in 

 Figs. 18, 19, and 20. 



The reader will have observed that we have referred to the 

 group under consideration in general terms, and without en- 

 deavouring to classify its members as plants or animals. And 

 indeed it is impossible to carry this great distinction down to the 

 lowest group of the organic world. This stands below the first 

 great forking of the tree of life ; its members remain in what has 



28 



FIG. 1 8. Organism of sleeping sickness 

 blood. The round bodies are red blood 

 corpuscles. 



Photo : F. Martin Duncan. 



