HOW PLANTS CAME TO DIFFER. 27 



line of plant-cells. This is the beginning of the 

 formation of the higher plants, which consist, in- 

 deed, of collections of cells, arranged either in 

 rows or in flattened blades, or many deep together 

 in complicated order. 



However, the higher plants differ from the 

 lower ones in something more than the number 

 and complexity of the cells which compose them. 

 They are very varied ; and their variety adapts 

 them to their special circumstances. For example, 

 desert plants, like the cactuses, have thick and 

 fleshy leaves (or, rather, jointed stems) to store 

 up water, with a very tough skin to prevent 

 evaporation. The flowers of each country, again, 

 are exactly adapted to the insects of that coun- 

 try ; and so are the fruits to the birds that swal- 

 low and disperse them. How did this all come 

 about ? What made the adaptation ? It is a re- 

 sult of two great underlying principles known as 

 The Struggle for Life, and Natural Selection. 



Since each early plant goes on growing and 

 dividing, again and again, as fast as it can, it 

 must follow in time that a great number of plants 

 will soon be produced, each fighting with the 

 others for air and sunlight. Now, some of them 

 must, by pure accident of situation, get better 

 placed than others ; and these will produce greater 

 numbers of descendants. Again, unless all of 

 them remained utterly uninfluenced by circum- 

 stances (which is not likely) it must necessarily 

 happen that slight differences will come to exist 

 between them. These differences of outline, or 

 shape, or cell-wall, may happen to make it easier 

 or harder for the plant to get access to carbonic 

 acid and sunlight, or to disperse its young, or to 

 fix itself favourably. Those plants, therefore, 



