220 PROGRESS OF SCIENCE IN THE CENTURY. 



ul(B from star-clusters, has afforded a method of 

 measuring the motions of stars in the line of sight, 

 and has led to many other results which afford fine 

 historical illustration of the value of co-operation 

 between sister-sciences. 



THE EVOLUTION-IDEA IN ASTRONOMY. 



The evolution-idea has asserted itself in astron- 

 omy especially in connection with what is called the 

 nebular hypothesis, an attempt to give an account 

 of the origin of a solar system. It is said to have 

 arisen as a transcendental conception in the mind 

 of Swedenborg; it was suggested on general grounds 

 by Kant; it was formulated in mechanical terms by 

 Laplace; and it has been the subject of much dis- 

 cussion on the whole unfavourable to its details, 

 though confirmatory of the general idea. 



It was in 1755 that Immanuel Kant (1724-1804) 

 published his General Natural History and Theory 

 of the Heavens, more than a quarter of a century 

 before his Critique of Pure Reason. Based, as its 

 title indicates, on Newton's Principia, the essay pic- 

 tures a possible mode of origin for the sun and the 

 planets from a homogeneous distribution of vaporous 

 particles in the space now occupied by the solar sys- 

 tem. 



A more important step was taken in 1706 when 

 Laplace presented his " Nebular Hypothesis." Start- 

 ing from a vast fluid nebula in slow rotation, he 

 supposed that as this cooled it contracted, that as it 

 contracted its rate of rotation increased, that event- 

 ually the "centrifugal force" of the great nebular 

 sphere exceeded the centripetal gravitational attrac- 

 tion, and a nebulous ring was separated off from the 



