PHYSICAL EVOLUTION 



CHAPTER I 



"During tlie (iuartcr of a century wliicli lia.s elapsed since 

 Biology began to occupy itself again with general problems, at 

 least one main fact has been made clear by tlie united labours of 

 numerous men of science, viz. the fact that the theory of descent, 

 the idea of development in the organic world, is the only con- 

 ception as to the origin of the latter which is scientifically tenable. 

 It is not only that, in the light of this theory, numerous facts 

 receive for the first time a meaning and a significance ; it is not 

 that, under its infiucnco, all the ascertained facts can be harmoni- 

 ously grouped together ; but in some departments it has already 

 yielded the highest results which can be expected from any 

 theory, it has rendered possible the prediction of facts, not indeed 

 with the absolute certainty of calculation, but with a high degree 

 of probability. It has been predicted that man, who in the adult 

 state only possesses twelve pairs of ribs, would be found to have 

 thirteen or fourteen in his embryonic state ; it has been predicted 

 that, in this early period in his existence, he would possess the 

 insignificant remnant of a very small bone in his wrist, the so- 

 called OS ccntrale, which must have existed in the adult condition 

 of his extremely remote ancestors. Both predictions have been 

 fulfilled, just as the planet Neptune was discovered after its 

 existence had been predicted from the disturbance induced in the 

 orbit of Uranus. 



" That existing species have not arisen independently, but have 

 been derived from other and mostly extinct species, and that on 

 the whole this development has taken place in the direction of 

 greater complexity, may be maintained with the same degree of 



