96 THE ADVANCE OF SCIENCE 



insectivorous plants, on the motions of 

 plants, pointed out the routes of explora- 

 tion which have since been followed by 

 hosts of inquirers, to the great profit of 

 science. 



Darwin found the biological world a 

 more than sufficient field for even his 

 great powers, and left the cosmical part 

 of the doctrine to others. Not much has 

 been added to the nebular hypothesis, 

 since the time of Laplace, except that the 

 attempt to show (against that hypothesis) 

 that all nebulce are star clusters, has been 

 met by the spectroscopic proof of the 

 gaseous condition of some of them. More- 

 over, physicists of the present generation 

 appear now to accept the secular cooling 

 of the earth, which is one of the corol- 

 laries of that hypothesis. In fact, at- 

 tempts have been made, by the help of 

 deductions from the data of physics, to 

 lay down an approximate limit to the 

 number of millions of years which have 



