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 scienti- 

 fically tenable. It is not only that, in the light of this theory, 

 numerous facts receive for the first time a meaning and signifi- 

 cance ; it is not only that, under its influence, all the ascertained 

 facts can be harmoniously grouped together ; but in some de- 

 partments 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 still with a high degree of probabilit}'. It has 

 been predicted that man, who, in the ^dult 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 

 existence had been predicted from the disturbances induced in 

 the orbit of Uranus. 



That existing species have not arisen independent!}', but have 

 been derived from other and mostly extinct species, and that 



