202 BEITISH 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 j r ear to 



