PRINCIPAL GROUPS OF PLANTS. 131 



greater degree to those progeny living under the same conditions, 

 and so gradually give rise to a different species. This, of course, 

 assumes that any change inducejd by environment would be trans- 

 mitted to the offspring, to be retained so long as the environment 

 remained constant, an assumption which .is probably not far 

 from the truth. While it is admitted that changes in the environ- 

 ment may cause direct responses, yet it is doubtful whether they 

 are definite or permanent enough to produce new forms.. Near 

 the end of the eighteenth century this explanation was supported 

 by Erasmus Darwin of England, St. Hilaire of France, and Goethe 

 of Germany. 



USE AND DISUSE. There is very little difference between this 

 explanation and the preceding one. Lamarck proposed, in the 

 early part of the nineteenth century, that the use or disuse of 

 organs would so modify them that the acquired differences would 

 be inherited by the offspring. But, here again, the proof depends 

 upon the transmission of acquired characters, and this is now 

 almost disproved. 



NATURAL SELECTION. In 1859 Darwin published his " Origin 

 of Species by Means of Natural Selection," and this single event 

 revolutionized science. In this book Darwin arranged an enor- 

 mous mass of facts gained through many travels, incessant obser- 

 vation, and prolonged experiments. He built up an argument in 

 such a convincing way as to immediately attract the attention of 

 the world, not only of scientists but of laymen. The theory of 

 natural selection has for its basis the idea that great competition 

 is continually taking place between individuals of the same species 

 and between the individuals of various species. This struggle for 

 existence results in the " survival of the fittest " and the destruc- 

 tion of the unfit. The idea that two plants or animals from the 

 same parent might vary slightly, suggested the belief that the 

 one which was better equipped for the struggle for existence 

 would survive and so transmit its desirable characteristics to its 

 offspring, and that the unfortunate one would not survive and 

 its undesirable characteristics would thus be lost to the race. 



The objections to the theory of natural selection are of various 

 kinds, but the most serious is probably the fact that it is hard to 

 conceive how a very slight difference in character can be of advan- 



