“ee 
HEREDITY AND EVOLUTION 263 
of some plants the older individuals of which have highly 
specialized leaves (Fig. 240), indicating that these plants with 
highly specialized leaves have been derived from ancestors with 
ordinary leaves and that in their development they pass through 
a stage resembling the adult condition of those ancestors. 
Geographical distri- 
bution. The present 
distribution of plants 
and animals can be ex- 
plained only in the light 
of evolution. 
Many plants have ex- 
cellent means of seed 
dispersal and are very 
widely distributed. Fre- 
quently these are recent 
and very successful spe- 
cies and give us little 
or no insight into the 
past history of plants. 
In general, however, 
high mountains or seas 
impose barriers which 
many plants and ani- 
mals cannot cross very Fic. 241. Eucalyptus forest of Australia 
readily if at all. Such 
species or groups of related species are often confined to one 
area or to a limited number of separate areas, and it is these 
species that give us the greatest information concerning the 
origin of plants and animals. 
From geological evidence we find that all the continents have 
had land connections in the past, and, moreover, that many land 
areas which at present are islands were formerly connected with 
continents or with other islands. The floras of different regions 
and islands frequently differ widely from each other, and in 
general this difference increases with the length of time during 

