PHYSICAL BACKGROUND OF EVOLUTION I5 



In 1943, C. von Weizsacker, after a study of recent infor- 

 mation concerning the distribution of matter outside the 

 stellar bodies, set up a theory which, in a modified form, re- 

 turns to the Laplace hypothesis. He thinks that when the 

 sun was formed by the condensation of interstellar matter, 

 a very large part remained on the outside as an envelope 

 from which the planets were formed. Development of this 

 theory is very ingenious and shows great promise although 

 it does not explain the formation of the sun itself. Also, the 

 theory has the added advantage of making it almost certain 

 that planetary systems surround nearly every star in the 

 universe. 



Still more recently, F. L. Whipple has taken this same 

 information concerning the distribution of interstellar mat- 

 ter and has developed what he calls the dust cloud hypothe- 

 sis, which attempts to explain the origin of the whole solar 

 system, the sun, and its planets. Whipple's explanation fits 

 into the modem pattern of astrophysics and may be the be- 

 ginning of our real understanding of the formation of the 

 stellar bodies. From an evolutionary point of view the the- 

 ory is most interesting, since it would produce innumerable 

 planets of all ages— born and still unborn in an endless series. 

 The vast masses of highly dispersed dust which are scattered 

 in space have been one of the highlights of recent astronomi- 

 cal discovery. These great clouds, which are composed of 

 hydrogen, helium, oxygen, nitrogen, carbon, and other ele- 

 ments, are now known to be undergoing a coalescing proc- 

 ess. The elements enter into combination to form minute 

 dust particles which are slowly forced into larger and larger 

 aggregates by gravity and the pressure of light from sur- 

 rounding stars. Whipple has worked out a very convincing 

 mechanics of light pressure and finally of gravity to show 

 how eventually, after upwards of a billion years, these huge 

 dust clouds condense to the point where pressure produces 

 the heat necessary to start the atomic fire of a sun. The 

 planets are formed by gravitational eddies which appear 



