en. v.] PLANETARY EVOLUTION. 385 



too much, tnerefore, to assert that the nebular hypothesis is 

 completely verified, like the hypothesis of gravitation. But 

 on the other hand, they understand little of the logic of 

 scientific inquiry who expect to obtain the same kind and 

 degree of evidence in the former case as in the latter. It 

 was part of Newton's rare good fortune that his hypothesis 

 was the generalization of a physical property of matter, 

 which could be verified by a single crucial instance. In 

 none of the concrete sciences can such kind of verification 

 be looked for. A theory relating to a heterogeneous assem- 

 blage of concrete phenomena can only be verified gradually, 

 as the successive groups of phenomena in question are one 

 after another successfully studied and interpreted. Thus the 

 complete verification of the nebular hypothesis, as applied 

 merely to the solar system, involves the complete explana- 

 tion of the chief dynamic and physical features of the 

 system ; and for this we have yet to wait. Meanwhile the 

 theory possesses such unmistakeable marks of genuineness, 

 it conforms in so many and various ways to the test of 

 reconciling the order of conceptions with the order of 

 phenomena, that no one capable of estimating scientific evi- 

 dence would hesitate in provisionally accepting it. Devised 

 to account for a certain limited group of phenomena, it not 

 only accounts for these, but also for other groups of pheno- 

 mena, not considered by its propounders. Facts which on a 

 superficial view appeared as obstacles to the theory, have on 

 closer examination turned out to be powerful arguments in 

 its favour. It is sustained by all the facts within our ken, 

 and invalidated by none. And it has so far thriven with 

 the progress of discovery during the past hundred and twenty 

 years, that at the present moment it commands wider assent 

 than at any previous time since its first promulgation. 



Of this last statement we find striking confirmation as 

 we pass beyond the limits of the solar system and seek for 

 evidence in the remotest depths of stellar space. It is well 



VOL. I. CO 



