Januaby 19, 1906.] 



SCIENCE. 



83 



as I believe Mr. Cook has done, we may for 

 purposes of illustration consider a genus 

 as a sort of archipelago of such islands, the 

 members of the group being separated by 

 rather shallow channels, while the archi- 

 pelago is itself separated from other similar 

 groups of islands by deeper channels. It 

 has been urged that a type-species, when 

 accurately defined, furnishes a sort of lati- 

 tude and longitude of one island in the 

 archipelago and thus gives one of the best 

 means of locating the group as a whole. 

 To a certain point this is undoubtedly 

 true, yet the information, however impor- 

 tant, is by no means sufficient for purposes 

 of classification, for it is obvioios that to 

 know the precise position of an island in 

 an archipelago gives no clue to the shape 

 and extent of the group. To acquire this 

 latter knowledge we must measure and 

 ^ sound the intervening channels. Let us 

 carry out our figure a little further and 

 agree that the breadth of a channel indi- 

 cates the extent of actual difference be- 

 tween the plants in question, while the 

 depth of the channel indicates the fixity of 

 this difference. It is obvious that islands 

 may be far apart and yet be divided only 

 by very shallow water, the space between 

 them being from its inconsiderable depth 

 very likely to be more or less filled with 

 spits, shoals or sand bars. Similarly, as 

 we all know, two species may in their 

 typical forms be very unlike, yet exhibit 

 such affinity that they are more or less con- 

 nected by frequent intermediates, hybrids 

 or atavistic forms. On the other, hand, two 

 islands may be very near together yet 

 separated by an exceedingly deep channel, 

 in which any intermediate bars, shoals or 

 islets are quite impossible; and in like 

 manner two species may exhibit close 

 habital similarity and yet maintain their 

 technical distinctions with perfect fidelity. 

 It is, of course, these deeper channels, 

 these natural intervals, no matter how nar- 



row and difficult to find, that should be 

 diligently sought as yielding the most satis- 

 factory limits of a genus. It is by the 

 relative or complete absence of intergrada- 

 tion that we must recognize differences of 

 high antiquity and profound classifieatory 

 significance. It is not by the visible extent 

 of the differences, but by their constancy 

 that their importance is to be measured. 

 This is, of course, no new thesis. It has 

 been reiterated, time and again, in one 

 form or another, by many distinguished 

 biologists. Yet it is a principle habitually 

 disregarded by many systematists, and to 

 its neglect is due a large part of the annoy- 

 ing diversity in current classification. 

 Large genera are daily being divided and 

 new ones artificially created on the basis of 

 differences which are regarded as impor- 

 tant solely from their magnitude or con- 

 spicuous character and with scarcely a 

 thought as to their constancy. But it is 

 easy to see that botanists, who confound 

 differences of degree with those of kind, or 

 lay great stress on the wide divergence of 

 certain tj^e species and fail to take into 

 account the species which are intermediate, 

 are like reckless mariners who estimate the 

 depth of a channel solely by its breadth. 



The great majority of new generic 

 propositions rest upon the examination of 

 a comparatively small number of species; 

 indeed, a considerable part of them are 

 brought out in works relating to some geo- 

 graphically restricted fiora. In such cases, 

 it is a practise all too common to treat 

 the newly proposed generic segregates as 

 though they were made up solely of their 

 representatives which chance to inhabit the 

 particular region studied. In many in- 

 stances this enables the writer to define his 

 genera with a specious definiteness, which 

 may appear very convincing to those whose 

 botanical activities are restricted to the 

 same local flora, but which, when viewed 

 in the light of a broader knowledge of re- 



