8 JOEL ASAPH ALLEN— CHAPMAN mEMOISS [$o?xxi, 



systematise Replying to an article in Science by Mr. Hubert Lyman Clark on the advisa- 

 bility of naming these slightly differentiated forms, he said: "We submit that the 'layman' 

 who is naturally so troubled and confused by the modern ways of finding out how and to what 

 extent animals are modified by their environment, is not the proper arbiter to determine the 

 value and bearing of expert knowledge. If in other fields of scientific research it is not demanded 

 that the investigator stop his work at the point where his results are within the comprehension 

 of the lay mind, why should the student of birds and mammals be expected to refrain from 

 extending his researches beyond the point of convenience for the layman ? " 



As an indication of this gradually changing estimate of the nature and importance of geo- 

 graphic variations, as well as of the increasingly great amount of material that passed through 

 his hands, it is interesting to observe that while from the beginning of his systematic studies in 

 1870 to 1890 Doctor Allen described only 16 species and subspecies of mammals, during the 20 

 years following 1890 he described 415. 



After treating at length of individual and climatic variations and illustrating Ms remarks 

 with many specific instances 2 which up to that time had escaped the attention of systematic 

 ornithologists, Doctor Allen presented an exposition of the "Causes of climatic variation," 

 to which, after 50 years, we can add but little. After calling attention to the fact that increase 

 in color is coincident with increase in humidity, and that the darker representatives of a species 

 occur "where the annual rainfall is greatest, and the palest where it is least," he added: " This 

 coincidence is clearly illustrated in the birds of the United States, when the darkest representa- 

 tives of a species, as a general rule (indeed without exception so far as known to me), come from 

 regions of maximum annual rainfall, and the palest from those of minimum annual rainfall." 



Part V of this classic work, "On the geographical distribution of the birds of eastern North 

 America, with special reference to the number and circumscription of the ornithological faunae," 

 has in reality a wider scope than the title implies, since it includes also a general discussion of 

 the laws governing the distribution of life, an outline of the primary natural history divisions 

 of the globe, and of the provinces of the North American temperate region. 



When we consider the comparatively limited, and often inaccurate, data which were 

 available 50 years ago, the soundness of the conclusions reached by Doctor Allen in his study 

 of these problems is a tribute to the breadth of his vision, the thoroughness of his methods, 

 and the excellence of his judgment. 



"I am aware," he writes, "of the diversity of opinions still prevalent among naturalists in 

 regard to the influence climate exerts on determining the geographical distribution of species 

 and that many writers on this subject attribute to it only a slight importance, or altogether 

 ignore it." That he, himself, did not share this view is evident when he adds: " That tempera- 

 ture is a powerful limiting influence affecting the range of species, especially in respect to their 

 northward and southward extension, is so easily demonstrable that I am surprised to see it 

 still questioned. I have myself subjected this principle to a rigid examination in studying the 

 distribution of the animals and plants of eastern North America, and have been surprised at 

 the exact coincidence I have almost constantly met with between their northern and southern 

 limits of distribution and isothermal lines, they following them in all their numerous undula- 

 tions, sweeping northward in the valleys and southward along the sides of mountain ranges. 

 The occurrence on isolated alpine summits of species existing at a lower level only far to the 

 northward, is of itself suggestive of the powerful influence temperature has on the distribution 

 of animals and plants. In the Northern Hemisphere a northern fauna and flora everywhere 

 extends along the mountains hundreds of miles to the southward of their respective limits in 

 the adjoining plains and valleys. Various other causes have, of course, a greater or less influence 

 in determining the range of species, but none other, on the land areas, humidity perhaps alone 

 excepted, is nearly so potent. The want of conformity of isothermal lines with parallels of 

 latitude has doubtless led to confusion in regard to this subject, since vain attempts have often 

 been made to circumscribe the botanical and zoological zones by the latter The iso- 



1 E. g., birds which subsequently were described as Pipilo erythrophthalmus alleni Coues, Ortyx virginianus floridanus Coues, Buteo lineatus 

 alleni Ridgw., Sturntlla magna argutula Bangs, etc. 



