216 Sinnott . — 7 /k? ‘ Age and Area ’ Hypothesis , drV. 
2. The following objections may be raised against this hypothesis : 
(a) Many effective factors other than age determine the area 
occupied by a species, notably physical and climatic barriers, the 
adaptability of species under different environments, the rapidity with 
which they may become dispersed, and the growth form to which they 
belong. 
(b) An analysis of various floras shows that the hypothesis 
necessarily implies that trees and shrubs are producing new species very 
much faster than are herbs, a conclusion against which there is much 
evidence. 
(< c ) The fact that herbaceous species have a much wider average 
range than woody ones necessarily implies that the herbaceous element in 
the vegetation of the world is more ancient than the woody element, a 
conclusion against which there is also much evidence. 
(d) ‘ Dying out ' of species is apparently taking place in many 
cases, both by actual extermination, which causes the last survivors to 
appear as ‘ relict ’ endemics ; and by the £ swamping ’ of isolated members 
of old species by crossing with newly developed forms. 
3. The various factors which determine the occurrence of endemism are 
discussed, and the complexity of the whole problem emphasized. 
