Living Organisms 747 



a "struggle for existence" and a consequent "survival of the fittest." 

 This permits a "natural selection" of the best to survive, and thus the 

 race as a whole is changed or benefited. If there were no natural selec- 

 tion or survival of the fittest through the various individual struggles for 

 existence, there would be present many of the weaker types from which 

 future populations might arise. In other words, the stronger win nat- 

 urally in their struggle with the less fit. This is a factor in the explana- 

 tion of the characteristics of a group of animals. 



3. Elmer's Theory of Orthogenesis (1898). — The theory of ortho- 

 genesis (or definitely directed evolution) suggested that the evolution 

 of organisms has followed a perfectly predetermined direction or path- 

 way; that complex organisms arose through a series of directed and or- 

 derly sequences from simpler forms, much in the same way that a com- 

 plex adult develops from the egg through a series of predetermined 

 stages. This theory is on the border line of a vitalistic or supernatural 

 interpretation of the directive physicochemical factors which cause evo- 

 lution. According to this theory, certain types of variations are naturally 

 destined to arise, and hence determine the course of evolution not merely 

 at random but along a definite or straight line. This theory attempts 

 to explain the origin of many characters which arise spontaneously with- 

 out visible or apparent causes. 



4. De Vries' Theory of Mutations (1901). — De Vries suggested that 

 the production of sudden mutations results in the appearance of pro- 

 found changes and differences between parents and offspring, thereby 

 producing new species. Natural selection operates to eliminate or retain 

 such organisms which have mutated. Undoubtedly some species have 

 arisen through mutation, as shown by tailless dogs and cats, the short- 

 legged breed of sheep (Ancon sheep) descended by mutation from a 

 long-legged ram, the hornless Hereford cattle descended from a single 

 calf born in Kansas in 1889. 



5. Weismann's Theories of the Continuity of Germ Plasm and the 

 Noninheritance of Acquired Characters. — One essential feature of Weis- 

 mann's doctrine is that the germ plasm (germinal material) is con- 

 tinuous or forms a direct path from one generation to the next and is 

 not derived from the soma or body plasm. Because of experimental evi- 

 dence, he maintained that characters acquired by the body plasm were 

 not inheritable. He suggested that only germinal variations which might 

 arise as a result of new combinations in the germ cells (independent of 

 environment) were inheritable. He recognized the almost limitless num- 



