202 BRITISH PLANTS 
The Evolution Theory. 
We have already drawn attention to the idea of evolu- 
tion in the case of vegetation inhabiting places where 
water or food is scarce. We showed that only plants 
which were specially modified or equipped for these 
conditions could successfully live in these regions. We 
pointed out that spines and prickles, and the defensive 
equipment of plants generally, have arisen gradually 
from variations which benefited their possessors and gave 
to them an advantage in the battle of life. Aquatic 
flowering plants were not always aquatics. Their an- 
cestors were land-plants which were driven into the 
water through stress of competition, and which gradually 
evolved variations more and more useful for an existence 
in water. In the evolution of the flower we showed that 
the line of development was a progressive adaptation to 
insect-visits for the purposes of pollination. 
The present statement of the Evolution theory is due 
to Charles Darwin, who died in 1888. The publication 
of his Origin of Species on November 24, 1859, forms 
a landmark in the long history of the progress of human 
knowledge. As applied to plants, it may be summed up 
as follows : The present races of vegetable forms have not 
existed unchanged, as we now find them, from the be- 
ginning. They are descended from ancestors which 
appear less and less like them as we recede into the past. 
The plant-remains, preserved to us in the rocks, prove 
this. Our present flowering plants are late arrivals on 
the scene. According to the theory, there has been a 
slow and gradual change of organisms, each a little better 
than its predecessor to live and multiply. Accompanying 
this advance there has been a remarkable increase in 
variety of form and specialization of structure. 
Malthus’ Law of Population. 
Plants, like animals, tend to multiply so rapidly that 
if there were no obstacles to its increase a single race 
might, in the course of time, overrun the whole earth. As 
a matter of fact, the number of individuals belonging to 
a particular race remains pretty constant from year to 
