346 Willis .— The Sources and Distribution of the 
remembered that every single genus in the whole flora would give such 
a diagram, reminding one of the ripples made by throwing a stone into 
water, and it seems to me difficult to maintain, in face of such facts, that the 
distribution of the species is not chiefly determined by a mechanical cause. 
This cause I believe to be simply age. Whether the genus be Monocotyle¬ 
don or Dicotyledon, tree, shrub, or herb, endemic or not, composed of wides 
and endemic species, or of endemics only, of Indo-Malayan, Australian, 
or South American affinity, it behaves in the same way. 
