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- 



