Mr. Herbert Spencer, in an essay (originally published in the Leader , March, 1852 , and republished in his Essays , in 1858 ), has contrasted the theories of the creation and the development of organic beings with remarkable skill and force. He argues from the analogy of domestic productions, from the changes which the embryos of many species undergo, from the difficulty of distinguishing species and varieties, and from the principle of general gradation, that species have been modified; and he attributes the modification to the change of circumstances. The author ( 1855 ) has also treated psychology on the principle of the necessary acquirement of each mental power and capacity by gradation.