THE PRINCIPLE OF NUMBER. 19 



changes, whether in the individual or in its descendants, 

 through the law of " symmetrical increase or decrease." By 

 this I mean that the number of sepals, petals, and stamens 

 often vary together from the typical number by the addition 

 or subtraction of a member. Thus, in a single corymb of 

 an Elder, 4-, 5-, 6-merous flowers may be often found ; simi- 

 larly, while early blossoming Fuchsias may bear 3-merons 

 flowers, they are replaced later by the regularly 4-merous 

 ones. Although these changes frequently occur in the same 

 plant, they usually are not permanent. Yet they occas,ionally 

 appear to haye become so, as in the terminal flowers of Adoxa 

 and Monotropa. On the other hand, the constant occurrence 

 and, therefore, specific character of 4-merons flowers in 

 Potentilla Tormentilla, and 3-merous in Tillcea muscosa, I 

 should be inclined to attribute to the fixation of a symme- 

 trical reduction which has taken place from the permanent 

 5-meroiis type so characteristic of Potentilla, and many 

 genera of the Grassulacece. Not infrequently the difference 

 of number is pronounced by systematists as generic; thus, 

 while Eubia has 5-merous flowers, Galium has 4-merous. A 

 similar difference lies between Buta and Saplophyllum.* 



If a cause be looked for, it would seem to be merely a 

 question of nutrition. If the symmetry varies in the same 

 plant, it is obvious that a corolla of four petals could not 

 have been pi-ovided with the same amount of nutritive 

 material as a 5-merous one. But if it be a specific character, 

 as in Tormentil (which, it may be observed, affectB the more 

 or less barren soil of heaths), then the change has become 

 fixed and is now hereditary. 



* By running the eye through the artificial keys at the commence- 

 ment of the Orders in the Oenera Plantarum of Bentham and Hooker, it 

 will be seen how frequently these authors regard the namber of parts in 

 the Calyx and Corolla as a prominent generic character. 



