172 DARWINISM TO-DAY. 



the needed variation can be produced through a slight ad- 

 vance or progressive change. Naturally it is not sufficient 

 if the variation appears in only a few isolated individuals, 

 but it is necessary, for the modification o-f the species, that 

 this variation occur in so many individuals that it will not 

 be extinguished through interbreeding but on the contrary 

 will be perpetuated. In other words selection works, except 

 in scattered cases where single or rare varia- 



Selection works 



with plural vari- tions are specially favoured by accidental iso- 

 lation, not with single but with pltiral variations 

 or varieties. Through this the host of variations is im- 

 mensely reduced as far as they come into consideration as 

 handles for selection, and of course only in this sense is 

 there any reason at all for the query as to whether we can 

 assume that the right variation will always be present at the 

 right time. The answer can only be, certainly not always : 

 many promising beginnings must always be checked in the 

 germ or at half-way, but in infinitely many cases the needed 

 plural variation will appear, because the same external 

 factors change a whole group of animals simultaneously 

 so that progress is possible. But, it goes without saying r 

 only a slow advance is conceivable on this basis. 



"One must not, for the rest, forget that the same condi- 

 tion of selective worth may be reached simultaneously 

 through combinations of different peculiarities, 

 value may be an< ^ that the same effect may be attained by 

 attained in dif- various means, both of which facts render it 



ferentways. . . 



importantly easier to get this selective worth. 

 When pursued by an enemy one individual of the harassed 

 species may save itself by a quick leap, a second through 

 sharp ears, a third by sharp eyes. Wallace rightly explains 

 that the necessity for a giraffe in times of famine is only to 

 reach as high as possible in the trees, and that different 

 means may avail for this, as a longer neck, long legs, or a 

 long tongue, all of which may eventually come to be correl- 



