INTRODUCTORY 5 



privet hawk-moth (Sphinx ligustri) must have a white line along each 

 side of the back. Ten years later, the English zoologist, Poulton, 

 succeeded in rearing the eggs of Sphinx ligustri, and it was then 

 demonstrated that the young caterpillar actually possessed the postu- 

 lated white lines. 



Such predictions undoubtedly give the hypothesis on which 

 they are based, the Evolution theory, a high degree of certainty, and 

 are almost comparable to the prediction of the discovery of the 

 planet Neptune by Leverrier. As is well known, this, the most 

 distant of all the planets, whose period of revolution round the sun is 

 almost 165 of our years, would probably never have been recognized 

 as a planet, had not Adams, an astronomer at the Greenwich Obser- 

 vatory, and afterwards Leverrier, deduced its presence from slight 

 disturbances in the path of Jupiter's moons, and indicated the spot 

 where an unknown planet must be looked for. Immediately all 

 telescopes were directed towards the spot indicated, and Galle, at the 

 Berlin Observatory, found the sought-for planet. 



We might with justice regard as lacking in discernment those 

 who, in the face of such experiences, still doubt that the earth 

 revolves round the sun, and we might fairly say the same of any one 

 who, in the face of the known facts, would dispute the truth of the 

 Evolution theory. It is the only basis on which an understanding of 

 these facts is possible, just as the Kant-Laplace theory of the solar 

 system is the only basis on which an adequate interpretation of the 

 facts of the heavens can be arrived at. 



To this comparison of the two theories it has been objected that 

 the Evolution theory has far less validity than the other, first, because 

 it can never be mathematically demonstrated, and secondly, because 

 at the best it can only interpret the transformations of the animate 

 world, and not its origin. Both objections are just : the phenomena 

 of life are in their nature much too intricate for mathematics to deal 

 with, except with extreme diffidence ; and the question of the origin 

 of life is a problem which will probably have to wait long for 

 solution. So, if it gives pleasure to any one to regard the one 

 theory as having more validity than the other, no one can object : but 

 there is no particular advantage to be gained by doing so. In any 

 case, the Evolution theory shares the disadvantage of not being able 

 to explain everything in its own province with the Kant-Laplace 

 cosmogony, for that, too, must presuppose the first beginning, the 

 rotating nebula. 



Although I regard the doctrine of descent as proved, and 

 hold it to be one of the greatest acquisitions of human knowledge, 



