90 SCIENCE PROGRESS 



It is not to be supposed, however, that the alternative 

 hypothesis is free from difficulties, some of which have been 

 slightly outlined, but we may still say that it gives a reason- 

 able explanation of many remarkable peculiarities. Further 

 evidence in its favour is no doubt wanted, as well as spectro- 

 scopic proofs of motions derived from the study of actual 

 existing spiral nebulae. Some recent work by Dr. Nolke on 

 the effect of a resisting medium in the evolution of the solar 

 system from a primitive nebulous condition has been published 

 by him at Berlin. Sir George Darwin in his article on ** The 

 Genesis of Double Stars" gives an interesting historical account 

 of work on the theory of the equilibrium of revolving liquid 

 bodies, by Poincare, Jeans and others, together with an applica- 

 tion of their results to stars of the Algol type. Probably there 

 is no subject more fascinating than the question as to the past 

 and future of our system ; and though from our limited experi- 

 ence, both in time and space, there is the greatest necessity for 

 caution in drawing conclusions, yet the mind of man seems so 

 constituted that it cannot help doing so. It remains for the 

 future to show whether " the vast masses of observational data 

 accumulated by the persevering industry of self-denying men of 

 science " can be put together in the manner indicated above to 

 yield the laws of stellar evolution. 



