it in the formation of genera, orders, or other groups, and when it is that we run the risk of being led astray by the too close adherence to the rules laid down.
The most important character, in plants, will always be that which in the greatest number of species (or groups of species) is the most constantly accompanied by the greatest general resemblances among those species, and differences from all others—that which collects into the same group species showing the greatest general conformity in the structure and economy of all their parts, and which may, therefore, be supposed to be the most uniformly influenced by or acting upon the specific constitution of plants.
This question of the relative importance of characters has been frequently discussed, especially by French botanists; and by none has it been so clearly put as by the elder De Candolle, in his admirable "Théorie Elémentaire." He there lays it down as a rule, that the value of a character is in the compound ratio of the importance of the organ it is derived from, and of the point of view in which that organ is considered.
But, in regard to the first element, how are we to determine the relative importance of organs? De Candolle indicates two modes: à priori, by the consideration of the functions they perform, or the part they take in the vital phenomena; à posteriori, by the observation of the extent to which they prevail, the number of species in which they exist. The former mode has been the one eagerly pursued or attempted by the greater number of generalizing botanists; the latter is that which, after all, has practically led to the best classifications; and though characterized by De Candolle as "très ingénieux mais peu applicable," is really that which he has himself followed in the best parts of his systematic works.
These two modes of argument correspond to those arguments from final causes, and from observation of facts, which have divided zoologists. But in plants we are much less able even than in animals to trace the modifications of form and structure to any final causes. The animal goes after, and selects his food; and the whole economy of his structure is modified according to the nature of that food, and where and how it is to be obtained. The plant is stationary and must take what food comes within its reach; and that food, and the mode of absorbing it, is very similar in all species; nor can we discover any other final cause why one set of plants, for instance, should always have alternate and another opposite leaves—why in Digitalis purpurea there should be on an average 1200 seeds fecundated and ripened for every two pair of stamens, whilst in several Acacias there should be 10,000 stamens to every head of flowers, which sets and ripens some half-dozen or a dozen seeds only. And yet characters like these are, in some instances, so constantly accompanied by so many general resemblances as clearly to distinguish natural groups of several thousand species.
The importance of organs, also, in another way, admits of two distinct qualifications, not always concurrent: physiological importance
