PHYSICAL EVOLUTION
CHAPTER I
"During the quarter of a century which has elapsed since Biology began to occupy itself again with general problems, at least one main fact has been made clear by the 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 conception 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 influence, all the ascertained facts can be harmoniously 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 centrale, 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