PRESENT STATUS OF THE QUESTION 21 



to a garden full of shrubs, which are continually 

 sending out new branches, the tips of which repre- 

 sent the plants and animals now existing; they grow 

 by their own internal force, but are kept within 

 limits by the action of natural selection, which 

 plays the part of the gardener with his pruning- 

 shears. Were it not for this pruning, the shrubbery 

 would speedily degenerate into a wild and formless 

 thicket. 



One of the earliest and most obvious objections 

 made against Darwin's theory was that a slight 

 favourable variation, arising in a few individuals 

 would be speedily swamped by cross-breeding with 

 the vastly more numerous individuals which did 

 not display that variation. To avoid this objection, 

 Moritz Wagner propounded his theory of the "origin 

 of species by separation in space," according to 

 which geographical separation of groups of indi- 

 viduals, by the prevention of interbreeding, pro- 

 duced new forms through a process of divergent 

 development. Everything that we know indicates 

 that geographical separation has been an important 

 factor in bringing about the diversity which char- 

 acterizes the living world, but few would attribute 

 to this factor the significance which Wagner gave 

 to it. 



Professor August Weismann of Freiburg (1834- 

 1914) was a most influential figure in the biological 

 controversies which raged in the latter part of the 

 nineteenth century, especially by his elaborate the- 



