344 HOW PLANTS LIVE AND WORK. 



The same method of reasoning may be applied to explain how 

 Primroses and Cowslips probably came into existence as separate 

 species. Let us admit that their many obvious points of resem- 

 blance indicate a fairly close blood-relationship, a descent along 

 slightly different lines from a common ancestor. We may call 

 this ancestor the Primula root-stock. As has been said, no plant 

 resembles its parents in every detail ; and it is easy to imagine 

 that while one individual finds an advantage from a certain small 

 variation, another is benefited by a slight departure from the 

 normal of a different kind. Thus it was that from the Primula 

 root-stock arose variations, increasing in intensity from generation 

 to generation, which gave rise on the one hand to the length of 

 flower-stalk characteristic of the Cowslip, and on the other culmi- 

 nated in the Primrose. 



It becomes possible, reasoning on the lines indicated above, to 

 explain to a very great extent the manner in which the various 

 beautiful contrivances possessed by plants have originated. It is 

 often noticed that cattle avoid certain grasses and other plants, 

 which one would suppose, on casual inspection, to be in every way 

 suitable for food. In many of these cases it is found that the 

 leaves are closely beset by fine hairs, and the food becomes very 

 unpleasant to chew. The reader may easily verify this for himself 

 by mixing some of the grass known as " Yorkshire Fog " in his 

 next salad. It is very probable that the ancestors of the " York- 

 shire Fog '' were by no means as hairy as their modern represen- 

 tative ; but if some of the plants chanced to develop a few hairs, 

 it is plain that they would be a little less likely to be devoured by 

 browsing cattle, and would consequently have more chance of 

 coming to maturity and setting their seeds. Their offspring would 

 inherit a tendency to be hairy, and those in which this trait was 

 most marked in their generation would be most likely to survive. 

 Thus it would gradually come to pass that the smooth-leaved 

 members would become extinct. 



It is also by such a gradual development and fixation of 

 favouring characters that weeds growing in agricultural crops 

 generally contrive that the ripening of their seeds shall take place 

 about the time the crop is gathered, in order that the seeds may 

 be sown with the corn. And just as we owe the lovely colours 

 and forms of flowers to the unconscious selection of insects, so 



