Jantjaby 26, 1906.] 



SCIENCE. 



145 



experience, had been found to be " particularly 

 pertinent and applicable to those geographical 

 ' subspecies,' ' races ' or ' varieties,' which have 

 become recognizable as such through their 

 modification according to latitude, longitude, 

 elevation, temperature, humidity- and other 

 climatic conditions." This has been till now, 

 and in general still is, the sense in which the 

 term subspecies has been employed and under- 

 stood by the large number of ornithologists 

 and mammalogists who constantly and sys- 

 tematically make use of it in designating 

 geographic forms that, while well-marked, are 

 known, or supposed, to intergrade. If we are 

 to take President Jordan literally, heat, cold, 

 humidity, aridity or other environmental con- 

 ditions, are merely factors ' modifying indi- 

 vidual development but not connected with 

 the origin of species.' It all depends, it is 

 claimed, 'upon whether or not the characters 

 shown by the forms commonly designated as 

 subspecies are 'actually hereditary.' If their 

 persistent transmission through practically 

 endless generations be not hereditary, there 

 seems necessary also a new definition for hered- 

 ity, as well as for subspecies. While the ac- 

 tion of all such influences is doubtless onto- 

 genetic, and is by many recognized as such, 

 any attempt to distinguish ' ontogenetic 

 species ' from other species, or subspecies, 

 tends to confusion of ideas rather than to any 

 useful discriminations. It may be that 

 President Jordan has failed to clearly express, 

 in the paragraph quoted near the beginning of 

 this article, the ideas he intended to convey, 

 for it seems to me — perhaps through some 

 special obtuseness on my part — that there is 

 lack of coherence, if not actual contradiction, 

 between the parts I have italicized and the 

 portion that intervenes. 



J. A. Allen. 



THE EVOLUTION OF SPECIES THROUGH CLIMATIC 

 CONDITIONS. 



The very interesting paper by Dr. J. A. 

 Allen (Science, November 24), under the 

 above title, like all really useful discussions 

 of evolution, inevitably suggests further ob- 

 servations. The facts presented are of the 

 highest importance, and for this very reason 



we want to be quite sure of them in every case. 

 When two ' subspecies ' are joined by inter- 

 mediates, the transition may be uniform all 

 along the line, or it may not. In the diagram 

 here given, the vertical line indicates differ- 

 ence of size; the horizontal one, of latitude; 

 and the ' curves ' are plotted in the usual way. 



mmm-A. 



The two more or less parallel lines indicate 

 the extremes of individual variation. Now if 

 'the variation in size from the north south- 

 ward is as gradual and continuous as the 

 transition in climatic conditions ' (Dr. Allen, 

 I. c, p. 664), the phenomenon will be expressed 

 by the dotted lines, except that in nature the 

 slope will never be quite so uniform, because 

 the change of climate is not perfectly uniform. 

 If, however (as is surely true of some of the 

 cases Dr. Allen cites), we have two practically 

 uniform subspecies, each true to its own type 

 within a certain area, but having between 

 them a region in which they completely inter- 

 grade, the curve will resemble the solid lines 

 of the diagram. The slight slope of the lines 

 from A to 5, and G to D, will be explicable as 

 the direct result of environment upon indi- 

 viduals. In such a case, it is clear that the 

 two subspecies, in the regions where they re- 

 main true, are in fact isolated from one an- 

 other, and that it is exactly where they are 

 not isolated, that they fail to conform to any 

 single definable type. Such a condition of 

 affairs might very well be produced if two 

 distinct forms had arisen in isolated places, 

 and their ranges had subsequently overlapped, 

 their evolution not having proceeded far 



