V. 



THE SIGNIFICANCE OF SEXUAL REPRODUCTION IN 

 THE THEORY OF NATURAL SELECTION. 



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 significance ; it is not only 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 abso- 

 lute certainty of calculation, but still 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 the embryonic state : it has been predicted 

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

 insignificant remnant of a very small bone in the 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 ex- 

 istence had been predicted from the disturbances 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 

 certainty as that with which astronomy asserts that the earth 

 moves round the sun ; for a conclusion may be arrived at as safely 

 by other methods as by mathematical calculation. 



If I make this assertion so unhesitatingly, I do not make it in 

 the belief that I am bringing forward anything new nor because 



