256 THE SIGNIFICANCE OP SEXUAL KEPRODUCTION 



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 state- 

 ment ' The organic world has arisen by development,' to the ques- 

 tion ' 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 un- 

 known 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 already 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 Niigeli 1 , 

 the famous botanist, has expressed decided doubts as to the general 

 applicability of the principle of natural selection. According to 

 Niigeli, the co-operation of the external conditions of life with the 

 known forces of the organism, viz. heredity and variability, are in- 

 sufficient 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, 

 1 but which is unable to create anything new : he believes that the 

 causes of transformation reside within the organism alone. Niigeli 

 further assumes that organisms contain forces which cause period- 

 ical transformation of the species, and he imagines that the organic 

 world, as a whole, has arisen in a manner similar to that in which 

 a single individual arises. 



Just as a seed produces a certain plant because it possesses a 

 certain constitution, and just as, in this process, certain conditions 

 must be favourable (light, warmth, moisture, &c.) in order that 

 development may take place, although they do not determine the 



1 C. Nageli, ' Mechanisch-physiologische Theoric der Abstammungslehre.' Miinchen 

 ii. Leipzig, 1884, 



