INTRODUCTORY STATEMENT 185 



of evolution is largely within the living organism itself. One must be 

 familiar with the units of life: cells, organisms, generations. One 

 must understand the methods of reproduction and the mechanisms 

 of heredity and variation resident in the cellular components of the 

 organism. Also, one must never forget that organisms live and grow 

 and reproduce only if in an appropriate environment and that the 

 environment has much to do with the expression of hereditary char- 

 acters. In addition, the environment acts as a guiding factor, direct- 

 ing the course of evolution along lines of fitness or adaption. The 

 highly varied character of the environment, moreover, tends to favor 

 a high degree of diversity in organisms and thus to give rise to a vast 

 multiplicity of different types. 



In view of all this, it will be necessary for the general student who 

 has had no previous training in biology to learn the fundamentals of 

 this subject before proceeding with the more specific materials of 

 genetics. 



THE MECHANISM OF EVOLUTION 



At the beginning of the present century very little was known about 

 the actual mechanism of evolution. We had Darwin's theory of nat- 

 ural selection, Weismann's theory of germinal continuity, and a 

 statistical knowledge of certain aspects of variation and heredity. 

 Something was known about the role of isolation in species-forming, 

 and the general fact of orthogenesis was appreciated. With the re- 

 discovery in 1900 of Mendel's work and the announcement by De Vries 

 of the mutation theory, genetics really began. Thirty odd years of 

 intensive research by hundreds of specialists have contributed so much 

 to our understanding of the workings of evolution that we now con- 

 sider that we know something about the main causes of evolution. 



Evolution is now looked upon as an extremely complicated process. 

 There is no one cause of evolution, as the extreme proponents of natural 

 selection once held ; rather, there are many causes, each acting upon and 

 in co-ordination with all the others. The mechanism of evolution is like 

 an intricate piece of machinery manufacturing a complex product. 

 Each part is geared up with other parts. Some parts are concerned 

 with feeding in the raw materials, others with separating and dis- 

 tributing the raw materials, others with assembling and shaping up 

 the various parts into something useful and with discarding defective 

 and useless parts, and still others with sorting out the different kinds 

 of products and keeping them in separate lots. 



