262 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



on the whole this development has taken place in the direction 

 of greater complexit}'^, 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 I think that any opposition will be encountered, but 

 simply because I wish to begin by pointing out the firm ground 

 on which we stand, before considering the numerous problems 

 which still remain unsolved. Such problems appear as soon 

 as we pass from the facts of the case to their explanation ; as 

 soon as we pass from the statement ' The organic world has 

 arisen by development,' to the question ' But how has this been 

 effected, by the action of what forces, by what means, and under 

 what circumstances ? ' 



In attempting to answer these questions we are very far 

 from dealing with certainties ; and opinions are still conflicting. 

 But the answer lies in the domain of future investigation, that 

 unknown country which we have to explore. 



It is true that this country is not entirely unknown, and if 

 I am not mistaken, Charles Darwin, who in our time has been 

 the first to revive the long-dormant theory of descent, has 

 alread}' given a sketch, which may well serve as a basis for the 

 complete map of the domain ; although perhaps many details 

 will be added, and many others taken away. In the principle 

 of natural selection, Darwin has indicated the route by which 

 we must enter this unknown land. 



But this opinion is not universal, and only recently Carl 

 Nageli^ the famous botanist, has expressed decided doubts as 

 to the general applicability of the principle of natural selection. 

 According to Nageli, the co-operation of the external conditions 

 of life with the known forces of the organism, viz. heredity and 

 variability, are insufficient to explain the regular course of 

 development pursued by the organic world. He considers 

 that natural selection is at best an auxiliary principle, which 

 accepts or rejects existing characters, but which is unable to 

 create anything new : he believes that the causes of trans- 



* C. Nageli, ' Mechanisch-physiologischc Thcorie dcr Abstammungs- 

 lehre.' Munchcn u. Leipzig, 1884. 



