456 GENERAL MORPHOLOGY OF PLANTS 



ings were first invented by early man. These formed the basis 

 for improvement and further invention, so that there has been 

 a gradual and natural growth up to the present highly efficient 

 state of things. This is evolution, i.e., it is the evolution of man 

 and his work from the savage state to the present time. History 

 teaches us this. But in parts of the earth man still exists in 

 the savage state. So there are half-civilized tribes where crude 

 and very inefficient appliances are still used. So among the 

 civilized nations there are shown various degrees of progress 

 toward the condition which has been reached in the most 

 highly civilized nations. The present savages, semi-civilized 

 tribes and less civilized nations have lagged behind in this 

 evolution. Some made little progress, while others progressed 

 for a time and then stood still. Thus we have, in different 

 parts of the world at the present time, living examples, which 

 represent some of the steps in this evolution of man from the 

 savage state. 



643. Evolution of an individual. In the development of an 

 individual it begins as a single cell. By growth and differen- 

 tiation of its parts it finally reaches maturity, completing its life 

 cycle, when single cells are formed which start another life cycle. 

 As an example let us take the fern plant. The asexual spore 

 from the spore case on the fern plant germinates and produces a 

 short thread, the protonema, or first thread, which resembles 

 some of the thread-like green algae. The end of this thread soon 

 begins to expand by growth and cell division. It now forms a 

 thin, flattened mass of cells, somewhat heart-shaped in form, 

 the prothallium. This resembles certain thin membranous cell 

 masses, or cell plates in the green algae, or the thallus of the liver- 

 worts. The sexual organs are next formed, and then the egg is 

 fertilized. The fertilized egg now divides, and in its early stages 

 recalls the very young embryo of the liverworts, but it soon 

 departs widely from the course of development shown by the 

 liverworts and mosses. Stem, roots and leaves are developed. 

 The prothallium which lived an independent existence dies, and 

 the fern plant becomes an independent plant able to carry on 



