HISTORY AND THEORIES OF EVOLUTION 389 



survivors, and they are transmitted by heredity to their off- 

 spring, while those having less favorable adaptations do not 

 survive and, hence, are eliminated. Thus did Darwin think 

 that species came into existence by a gradual improvement 

 of advantageous variations until the type was best adapted 

 to its surroundings. "I am convinced," he said, "that natural 

 selection has been the main though not the exclusive means of 

 modification." 



One of the many objections offered against this theory is the 

 perpetuation and improvement of disadvantageous modifica- 

 tions, such as the beautiful colors and songs of birds and adap- 

 tations for fighting which render them conspicuous. Darwin 

 explained many of these cases by his theory of sexual selection. 

 Among the higher animals it is a fact of common observance 

 that in mating the members of either sex prefer the most at- 

 tractive individuals of the opposite sex. The successful rival 

 wins the mate, and, of course, it is his characteristics which are 

 transmitted and improved in succeeding generations. Adap- 

 tations for rivalry by battle among males are explained in the 

 same way. 



Among the helpful contemporaries of Darwin several must be 

 mentioned. To Herbert Spencer (1820-1903), the philosopher 

 and author of "Principles of Biology," we owe the phrase, 

 "the survival of the fittest." Haeckel summed up the recapit- 

 ulation theory of Von Baer (published in 1828) i. e., that the 

 embryonic phases of higher forms resemble or pass through the 

 corresponding embryonic stages of lower forms in his funda- 

 mental law of biogenesis, that ontogeny recapitulates philogeny. 

 Huxley was the author of "Man's Place in Nature." 



Dr. August Weismann (born in 1834) is the foremost oppo- 

 nent of Lamarck as to transmission of acquired characters. He 

 led to the critical examination of reported cases, and he claims 

 that no case really shows the transmission of acquired char- 

 acters. He recognizes the chromatin as the hereditary sub- 

 stance or idioplasm, and calls the idioplasm of the germ cells 

 germ plasm. This germ plasm, he says, is "never formed de 

 novo, but it grows and increases ceaselessly ; it is handed on from 

 one generation to another like a long root creeping through the 

 earth, from which at regular distances shoots grow up and be- 



