September 1, 1905.] 



SCIENCE. 



261 



finally replaced by a planet with attendant 

 satellites. 



The whole process forms a majestic pic- 

 ture of the history of our system. But the 

 mechanical conditions of a rotating nebula 

 are too complex to admit, as yet, of com- 

 plete mathematical treatment ; and thus, in 

 discussing this theory, the physicist is com- 

 pelled in great measure to adopt the quali- 

 tative methods of the biologist, rather than 

 the quantitative ones which he would pre- 

 fer. 



Although the telescope seems to confirm 

 the general correctness of Laplace's hy- 

 pothesis, yet it is hardly too much to say 

 that every stage in the supposed process 

 presents to us some impossibility. 



Thus, for example, the ring of Saturn 

 seems to have suggested the theory to La- 

 place; but to take it as a model leads us 

 straight to a quite fundamental difficulty. 

 If a ring of matter ever concentrates under 

 the influence of its mutual attraction, it 

 can only do so round the center of gravity 

 of the whole ring. Therefore the matter 

 forming an approximately uniform ring, 

 if it concentrates at all, can only fall in on 

 the parent planet and be reabsorbed. 

 Some external force other than the mutual 

 attraction of the matter forming the ring, 

 and, therefore, not provided by the theory, 

 seems necessary to effect the supposed con- 

 centration. The only way of avoiding this 

 difficulty is to suppose the ring to be ill- 

 balanced or lop-sided ; in this case, pro- 

 vided the want of balance is pronounced 

 enough, concentration will take place round 

 a point inside the ring but outside the 

 planet. 



However, this is not the time to pursue 

 these considerations further, yet enough 

 has been said to show that the nebular hy- 

 pothesis can not be considered as a con- 

 nected intelligible whole, however much of 

 truth it may contain. 



In the first theory which I sketched as 



to the origin of the sun and planets, we 

 supposed them to grow by the accretions of 

 meteoric wanderers in space, and this hy- 

 pothesis is apparently in fundamental dis- 

 agreement with the conception of Laplace, 

 who watches the transformations of a con- 

 tinuous gaseous nebula. I must not pause 

 to consider how these seemingly discordant 

 views may be reconciled, but will merely 

 say that I conceive both theories contain 

 important elements of truth. 



"We have seen that, in order to explain 

 the genesis of planets according to La- 

 place's theory, the rings must be ill- 

 balanced or even broken. If the ring were 

 so far from being complete as only tp cover 

 a small segment of the whole circumfer- 

 ence, the true features of the occurrences 

 in the births of planets and satellites might 

 be better represented by conceiving the 

 detached portion of matter to have been 

 more or less globular from the first, rather 

 than ring-shaped. Now this idea intro- 

 duces us to another group of researches 

 w^hereby mathematicians have sought to ex- 

 plain the birth of planets and satellites. 



The solution of the problem of evolution 

 involves the search for those persistent or 

 stable forms which biologists would call 

 species. The species of which I am now 

 going to speak may be grouped in a family, 

 which comprises all those various forms 

 which a mass of rotating liquid is capable 

 of assuming under the conjoint influences 

 of gravitation and rotation. If the earth 

 were formed throughout of a liquid of the 

 same density, it would be one of the species 

 of this family ; and indeed these researches 

 date back to the time of Newton, who was 

 the first to explain the figures of planets. 

 - The ideal liquid planets we are to con- 

 sider must be regarded as working models 

 of actuality, and inasmuch as the liquid is 

 supposed to be incompressible, the condi- 

 tions depart somewhat widely from those 

 of reality. Hence, when the problem has 



