268 THE POPULAR SCIENCE MONTHLY 



serious confusion. In view of all the evidence then which must be 

 taken into account, it is clear that the final decision on the validity of 

 any group must be rendered by the systematic workers in the larger 

 group to which it belongs. It is not to be expected that physiologists 

 or experimental zoologists or even entomologists can decide as to either 

 the validity of a species of sea-urchin or the desirability of a genus 

 of birds. And this leads to a third principle often overlooked. 



While genera and larger groups in our systems of classification 

 ought to be based on relationship, their delimitation is often of neces- 

 sity artificial and is purely a matter of expediency or convenience. 



The English zoologist Duncan maintained that it is " impossible 

 to admit genera which are not differentiated by characters which have 

 a decided and important physiological value." Other zoologists have 

 ignored or distinctly repudiated this view. The difference of opinion 

 seems to be based on a difference in the approach to the question and 

 it is to this same difference of approach that all discussion as to the 

 relative merits of large and small genera is due. The zoologist who is 

 particularly occupied with the diversity of species and who has ex- 

 amined large numbers of individuals in the attempt to establish 

 specific limits becomes impressed with the great importance of con- 

 stancy in any given character and, when he finds a group of species 

 which possess in common a character, or certain characters, con- 

 stantly maintained, he finds it desirable to designate them by a com- 

 mon name and the group is to him a genus. Such a process naturally 

 leads to numerous small genera. On the other hand, the zoologist who 

 studies, from the morphologist's view point, relatively few specimens, 

 representing perhaps many species, is naturally most impressed by the 

 resemblances, and he finds that as regards characters which to him 

 seem of physiological importance, his material divides into compara- 

 tively few groups. These he designates as genera and since the minor 

 characters reveal a more or less marked differentiation into species his 

 genera are naturally large. It seems to me that the difficulty is the 

 same as would arise were one trying to decide what is a branchlet on a 

 tree. In certain old, nearly dead trees, or in very young ones, there 

 might be little difficulty, main branches, secondary branches, branch- 

 lets and twigs would all be sufficiently few to be distinguishable and 

 there would be little disagreement as to the limits of the branchlets. 

 But in most trees this would not be the case and much would depend 

 on whether one began at the twigs and worked downward or at the 

 branch and worked upward. In other words, while genera are, or at 

 least ought to be, natural groups, their limits are often necessarily 

 artificial and arbitrary. "We recognize them by name for convenience 

 and their limitation is largely to be determined by expediency. To 

 say, as Duncan does, that it is " impossible " to recognize certain 



