THE HISTORY OF SCIENTIFIC DISCOVERY. XV 



opens so extended a range of new investigations, and contrasts so 

 strongly with the complexities and incongruities of the older doc- 

 trines, as to leave little liberty of choice between the opposing theo- 

 ries. Not only do the reception of these views mark a signal epoch in 

 the progress of science, but from their comprehensive bearings and 

 the luminous glimpses which they open into the most elevated re- 

 gions of speculative inquiry, they have a profound interest for 

 many thinkers who give little attention to the specialties of exact 

 science. 



In the history of human affairs there is a growing conception 

 of the action of general causes in the production of events, and a 

 corresponding conviction that the part played by individuals has 

 been much exaggerated, and is far less controlling and permanent 

 than has been hitherto supposed. So also in the history of science 

 it is now acknowledged that the progress of discovery is much 

 more independent of the labors of particular persons than has been 

 formerly admitted. Great discoveries belong not so much to indi- \ 

 viduals as to humanity ; they are less inspirations of genius than j 

 births of eras. As there has been a definite intellectual progress, 

 thought has necessarily been limited to the subjects successively 

 reached. Many minds have been thus occupied at the same time 

 with similar ideas, and hence the simultaneous discoveries of inde- 

 pendent inquirers, of which the history of science is so full. Thus 

 at the close of the sixteenth century, philosophers had entered 

 upon the investigation of the laws of motion, and accordingly we 

 find Galileo, Benediti, and Piccolomini proving independently that 

 all bodies fall to the earth with equal velocity, whatever their size 

 or weight. A century after, when science had advanced to the 

 systematic application of the higher mathematics to general phys- 

 ics, Newton and Leibnitz discovered independently the differential 

 calculus. A hundred years later questions of molecular physics 

 and chemistry were reached, and oxygen was discovered simulta- 

 neously by Priestley and Scheele, and the composition of water by 

 Cavendish and "Watt. These discoveries were made because the 



