THE PAST HISTORY OF PLANTS. 205 



For example, all the peaflower family are dis- 

 tinguished by their possession of a peculiar blos- 

 som whose corolla consists of a standard, a keel, 

 and two wings, like sweet-pea or broom. This 

 family contains several genera, one of which is 

 that of the clovers, including certain peaflowers 

 which have learned to mass their blossoms into a 

 roundish head, and have trefoil leaves, and very 

 few seeds in the short seed-pod. The clovers, 

 again, are subdivided into species or kinds, such 

 as purple clover, Dutch clover, hop clover, and 

 hare's-foot clover ; in Britain alone, we have 

 twenty-one such distinct species or kinds of clo- 

 ver. You will see at once that this method of 

 grouping by ancestral forms enables us largely to 

 reconstruct the history of each particular plant 

 or animal. 



Why don't these kinds cross freely with one 

 another, and so produce an endless set of puzzling 

 hybrids ? Well, they do occasionally ; and such 

 mongrel forms often show us every possible vari- 

 ation between the two parents. But this can only 

 happen when the parent stocks are very close to 

 one another ; and even then, the hybrids tend to die 

 out rapidly. Why ? Because each of the parents 

 is better adapted to a particular situation; the 

 hybrid usually falls between two stools, and gets 

 killed down accordingly. It cannot stand the 

 competition of the true species. New kinds, how- 

 ever, may sometimes take their rise from chance 

 hybrids, which happen to possess some combina- 

 tion of advantages. 



Thus plants in the mass, as we see them 

 around us at the present day, are divisible into 

 several well-marked groups, some of which are 



