CHAPTER XVIII 



THE LOWEST ORGANISMS AND THE CELL AS THE 

 LIFE UNIT 



192. The process of evolution. The higher complex animals 

 and plants are readily distinguished from one another, but the 

 differences become less apparent in the lower, simpler forms. 

 There are indeed groups of uncertain position, some authors 

 placing them among the plants and some among the animals. 



The animal and plant kingdoms, in the process of evolution, 

 followed a tree-like method of development. The forms and 

 groups split up into divergent lines which constantly gave off, 

 and are still giving off, new shoots. Thus from a number of 

 trunks in the beginning there have been derived a multitude 

 of smaller branches, and from these in turn have arisen count- 

 less twigs. It is impossible to construct accurately these genea- 

 logical trees, because the species now living -occupy the position 

 of buds on the structure, some relatively low down and some at 

 the highest points, but all at the ends of their respective lines 

 of development. The forms which represented the lowest and 

 intermediate stages of development are almost all extinct, that 

 is, have long ago died out on the earth, and we can only judge 

 of their structure by the fragmentary remains which are left as 

 fossils, or by comparative studies on the structure and develop- 

 ment of living species, which frequently give us suggestions of 

 what took place in the long process of organic evolution. 



193. The simplest living unit a cell. The living material of 

 organisms, that is the part which possesses life, is called pro- 

 toplasm. Protoplasm is not a simple substance, but, on the 

 contrary, is the most involved mixture of the most complex 

 substances which the chemist knows. These belong to the group 



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