36 



PAIRED SPECIES. 



We have to consider the status of the so-called " paired species," that is 

 to say, species from neighbouring or widely separated areas, with the same 

 habits and so closely similar in structure that a large section of the individuals 

 are indistinguishable, while other individuals at the opposite ends of the series 

 are easily separated by morphologic characters. We have, for example, a series 

 of a species found in the spruce forests of northern British Columbia whose 

 variations in certain characters intergrade from A to G, and a second series from 

 Engelmann's spruce of the Rocky Mountain Region of southern British Columbia 

 and Alberta with the same habits and the same characters as the first series, 

 but with the variations in the characters already chosen varying from B to H. 

 Individuals from either region lying between the points B and G on the curve 

 are indistinguishable from those of the other region lying between the same 

 points, while it is more or less easy to distinguish those about A from those 

 about H . 



A B G H 



There are apparently two explanations for such a condition. We may 

 have to deal with two distinct species which have arisen from distinct parent 

 forms and which through convergence have now come to intergrade; or we 

 may have to do with a single species, all descendants of a common parent species 

 which have varied in different directions while intergrading in the same characters 

 throughout the great bulk of the individuals. Either of these two hypotheses 

 may be correct, but as the second appears to be far the simpler and at the same 

 time " practical," it should be preferred. 



Classification has two objects; to express biologic facts, and to assist in our 

 study of the organisms themselves. It is more important to express the facts, 

 if we can be reasonably sure of them; but if the truth is very doubtful, as it 

 certainly is in the case of these " paired " or " overlapping " species, it appears 

 to the writer that the simpler and more useful explanation should be chosen. 

 If the individuals about A and H were the only ones known, we should describe 

 two species; and it is proper to do so, if good judgment is used and the description 

 sufficient, for it is only by the publication of such studies that rapid progress 

 can be made. If intergrading forms are discovered later, and a few names 

 reduced to synonymy, we still have a valuable record of variations, and synonyms 

 are surely less troublesome than composite species. But when we find many 

 intergrading individuals in each series so that the two series overlap and much 

 of the material cannot be determined without the place labels, the writer can 

 see no sufficient reason in retaining two names. In this paper several series 

 in which no constant and distinguishing characters appear are left under their 

 several names awaiting the completion of breeding experiments. 



PLATE 8. 

 IPID STRUCTURES ALL MUCH ENLARGED; ORIGINAL. 



Fig. 1, Trypodendron retusus Lee.; larva. 



Fig. 2, Pityogenes knechteli Sw. ; male genitalia. (See Fig. 3.) 



Fig. 3, Orthotomicus caelatus Eichh.; male genitalia; Ap., apodemes; cc., central cavity; d.p., 



dorsal plate; Sp., spicule; Teg., tegmen; Tr., trough; Vp., ventral plate. 

 Fig. 4, Anisandrus populi Sw.; larva. 



Fig. 5, Egg-tunnels of Conophthorus coniperda Sz. in white pine cone. 

 Fig. 6, Eccoptogaster picece Sw.; pupa. 



Fig. 7, Anisandrus pyri Peck; male, side view (A. E. Kellett). 

 Fig. 8, Trypodendron retusus Lee.; pupa. 



