234 



NA TURE 



[January 8, 1903 



present more detail, are brighter, and appear larger than when 

 seen through an ordinary single telescope of the same aperture. 

 Tn addition to this, the "seeing" is much steadier, and the 

 stereoscopic effect obtained greatly enhances the beauty of the 

 objects observed. 



Such objects as Clark's companion to y Lyra:, the companion 

 to t Orionis and the Mitchell companion to Rigel have all 

 been steadily observed, and it is generally considered neces- 

 sary to use an instrument of 7 or S inches aperture in order 

 to see the last-named object. 



The prisms used in this instrument are i\ inches long and 

 i n :hes thick, the rays from the objectives traversing 5* inches 

 of glass before reaching the eyepieces. 



RECENT AMERICAN BOTANY. 



\ [ R. M. L. FERNALD > has published a very interesting 

 review of the birches belonging to the groups Betula 

 ;.' i and B. nana. These trees and shrubs inhabit the northern 

 regions of both hemispheres, and Mr. Fernald recognises in 

 America seven species and seven varieties, of which six species 

 and five varieties are common to the Old World. Thus, 

 contrary to the opinion of some recent authors, the American 

 white birches are mostly non-endemic, though exhibiting 

 numerous apparently distinct forms. Not only is this true, but 

 the admitted species intergrade all along the line. " It is quite 

 possible to trace by a series of specimens a direct connection 

 between the dwarf Betula nana or B. glandulosa and the tall 

 B. alba. . . . But since it is obviously impracticable to regard 

 all these forms as one species, it seems wiser to recognise the 

 more marked centres of variation as species which are admitted 

 to pass by exceptional tendencies to other forms ordinarily 

 distinguished by marked characteristics" (p. 189). This, of 

 course, brings up the question of the definition of species. The 

 present writer has been accustomed to use the accompanying 

 diagram in teaching biology. The line a a represents a species 

 which is slightly dimorphic, as is indicated by the two promin- 

 ences. The line b b represents a strongly dimorphic species, 

 connected (at b') by very few intermediates. The line < c 

 represents a case in which the intermediates have died out, and 

 there is a complete break (at 1 •') resulting in the formation of 

 two species. It is now to be pointed out that this break must be 

 spicial or geographical, and not merely morphological, otherwise 

 the two sexes of the same species would often have to be regarded 

 as distinct species. Such a break need not be geographical in the 

 ordinary sense, but when the two species inhabit what is nomin- 

 ally the same locality, they are found to be differently related to 

 their environment, or related to different closely adjacent 

 environments. Furthermore, they must breed true, and not 

 ordinarily interbreed one with another. 



This sounds simple enough, but the application of these 

 principles is not so simple. In the diagram, the case of /' b is 

 obviously more like that of re than it is like that of a a. The 

 difference between a slight break and a slight connection is 

 infinitesimally small, yet after all it is a real difference — some- 

 thing existing in Nature, and not subject to individual opinion. 

 If this criterion is admitted, because of its capability of exact 

 definition, then the whole series of birches discussed by 

 Mr. Fernald must apparently be regarded as one species ! 



Another sort of case is offered by the plants of the Galapagos 

 Islands, recently reviewed in a. most valuable memoir by 

 Dr. B. L. Robinson.- Euphorbia viminea^ J. D. Hooker, has 

 eight distinct forms confined to as many islands (one only being 

 found on two). These plants are readily distinguishable, but 

 their characters are such as would be ordinarily of no value for 

 distinguishing species in the genus. On continental areas, similar 

 species of Euphorbia are polymorphic, with innumerable similar 

 variations connected by every sort of intermediate. Conse- 

 quently, Dr. Robinson does not treat the Galapagos plants as 

 separate species, or (with one exception) even as varieties, but as 

 "forms." Now, according to the above definition of species, 

 these plants are perfectly good species, for the breaks in con- 

 tinuity, slight as they are, appear to be absolute. 



There is, perhaps, one way of escaping from this conclusion. 

 Distinct species should not promiscuously interbreed ; there 

 should be some sort of " physiological " barrier. It is known, 

 in 1 he case of the ostensibly distinct species of Lavatera from the 



1 Amer. Journ. Science^ xiv., Seprember, 1902. 



. Amer. Acad., October, 1902 (vol. xxxvtii.). 



islands oft" the coast of California, that this barrier does not 

 exist. Perhaps, if the different Galapagos Islands' forms of 

 Euphorbia viminea were grown together, they would completely 

 fuse and give a single promiscuously varying type like those 

 of the continents. But, after all, the question is what they 

 actually do, not what they might do, undei hypothetical 

 conditions. The answer to this question must be that they 

 remain distinct. 



It seems to the present writer that the only precise criterion 

 of species must be a spacial one, just as the only reason for species 

 is that of function, or the relation between the nature of the 

 creature and the place it occupies. But, admitting this on 

 philosophical grounds, we are forced to recognise species of 

 every degree of distinctness, just as the geographer recognises 

 islands separated by every sort of distance from the mainland. 

 It is easier, no doubt, to accept instead the morphological 

 criterion, and this is actually what we have to do in taxonomic 

 w.nk, 1 for lack of evidence of the other kind ; but this leaves 

 the whole matter to be decided by individual opinions, with 

 results known too well. 



It is probable, if not ceitain, that variable plants on continental 

 areas produce many "temporary species." That is to say, 

 local colonies become more or less differentiated and remain so 

 until swamped by invasions of the parent form or some other 

 variety. Whether we recognise these "temporary species" 

 depends, in practice, upon the degree of difference exhibited. 

 Not rarely, the disiinclions are constant and marked over a 

 certain area, but the very same distinctions elsewhere occur as 

 individual variations in the midst of the parent species. I have 

 recorded such cases in the genera Spha-ralcea and Cleome. 



At the close of his work on the Galapagos flora, Dr. Robinson 

 presents a most lucid and philosophical discussion of the whole 

 subject ; it is so full of fact and thought that a brief summary 

 could not do it justice. In particular, attention must be called 



to his statement of the reasons why the local insular varieties 

 persist in spite of the occasional infusion of new blood. 



Mr. Carl Purdy's revision of the genus Calochortus- is another 

 work of great interest. These beautiful " butterfly lilies " are 

 extremely abundant in the Pacific region of North America, and 

 are almost indefinitely variable. The variations are of all sorts, 

 sometimes "constitutional" rather than morphological. Says 

 Mr. Purdy, "In cultivation it has frequently been found that a 

 very slight variability in strains is accompanied by a marked 

 constitutional difference. In two beds of Calochortus venuslus, 

 planted in the same soil and separated only by a thin board, it 

 would puzzle a botanist to state wherein the plants vary. They 

 come from widely separated localities, and the difference is one 

 more easily detected by the eye than conveyed by words. In 

 one bed, two-thirds of the leaves are already destroyed by 

 mildew (Botrytis), while in the other, not one leaf is injured ; 

 and such is the case whenever and wherever the two are planted " 

 (p. 10S). Mr. Purdy points out that in some localities the 

 plants are very uniform, while in others they are extremely 

 variable, with hundreds of distinguishable phases. It is 

 probable that the phenomenon of "temporary species" is 

 common in this genus, and the union of such morphologically, 

 but not physiologically, distinct types is the cause of much 

 variability. At the same time, there are species which always 

 remain distinct, never producing fertile hybrids. That Mr. 

 Purdy has tested so many of the forms for such "physiological 

 barriers " gives his work especial value and importance. It does 



1 De Vries has assumed that, because botanists so distinguish species 

 (admittedly of necessity), therefore the morphologic al criterion is the genuine 

 one. Thus species have no better foundation in Nature than genera, which 

 are wholly based on reasons of convenience. 



- Proc. Calif. Acad. Sci., 3rd series. Botany, vol. ii. No. 4 (1901). 



NO. 1732, VOL. 67] 



