NATURAL SELECTION 147 



of scattering their seeds to a distance if the species is not to 

 be exterminated. To effect this, various agencies are available. 

 There is the wind, there are animals, and of animals there are 

 various kinds. The plant may by a lucky coincidence establish 

 a rapport between itself and any of these : until the rapport has been 

 established, it is free to choose it is hard to avoid this incorrect 

 word. No sooner, however, has it been established than the 

 freedom of choice is gone. Competition with rivals necessitates 

 further improvement along the line adopted. It is too late in the 

 day to begin another policy, to start de novo, when the competitors 

 have advanced far beyond the rudimentary stage. All this becomes 

 clear when we consider some very familiar seed-scattering con- 

 trivances. Among those that work in connection with the wind are 

 the samara, the fruit of the maple (Acer campestre), and the thistle- 

 down, both excellent; also there is the little round gourd of the 

 colocynth, which the wind-storms send bowling over the sands of 

 the Sahara. Appealing to birds to carry them are strawberries and 

 blackberries, and scores of others ; appealing to woolly or hairy 

 quadrupeds are burs. For the more primitive plants, from which 

 the elaborate forms of our day have sprung, the dispersal of seed 

 was a problem of which there were many possible solutions. 

 Now for each elaborated species there is only one. It began to 

 specialise long ago, and is bound to keep to its old methods. 



There are other cases where we see a variety of ways of Variety of 

 effecting a rapport with exactly similar environments. The hop- ^ Sl ^ 

 plant, the white briony, ivy, all make use of trees or shrubs to tions to 

 support them. But their methods are very different : the hop Ifailar 

 twines its stem round its prop, the briony holds by its tendrils, environ- 

 the ivy by little rootlets. Hungry animals are part of the en- m 

 vironment to which plants have to adjust themselves. And they 

 adopt various means of defence (it is hard to avoid speaking as 

 if they acted with a purpose in view). They become nauseous 

 or thorny. And the thorns or spines are of various kinds. They 

 may be really branches proving this, little leaves may be seen 

 on the spines of the hawthorn or hardened cells as in the brier, 

 or stiffened veins of leaves as in the holly. For many butterflies, 

 birds are an important part of the environment. They may adjust 



