1840
 
  Martin Barry expressed the belief that the spermatozoon enters the egg.
 
1840
 
 
 
 
1841 William Henry Harrison becomes ninth president of the United States; dies one month after inauguration.

John Tyler becomes tenth president of the United States.
 

 
 
 
1842
 
 
  John Stuart Mill publishes Logic.
 
1843 Congress grants S. F. B. Morse $30,000 to build the first telegraph line (Washington to Baltimore).

Sequoyah, Cherokee Indian leader, creates Cherokee syllabary and develops the first written form of a native North American language.
 

 
  Darwin first outlines his thoughts on natural selection in an unpublished essay.
 
  Chambers publishes (anonymously) his Vestiges of the Natural History of Creation.
1844
 
 
 
 
1845 James K. Polk becomes eleventh president of the United States.
 
 
 
 
1846
 
 
 
 
1847
 
 
 
 
1848
 
 
 
 
1849 Zachary Taylor becomes twelfth president of the United States.
 
 
 
1850