January 8, 1915] 



SCIENCE 



67 



way, result from environmental differences. 

 But, in this instance, " the climatic features 

 (zonal and faunal, as well as associational) 

 are identical on the two sides of the river." 

 Therefore, it is "reasonable to presuppose 

 separate and rather remote centers of differ- 

 entiation, and convergent dispersal through 

 time and space which brought the resulting 

 types to the verge of the river, beyond which 

 they were unable to spread." It is needless to 

 point out that hypotheses exist, e. g., that of 

 " mutation," which do not invoke the aid of 

 environmental differences to account for all 

 specific change. According to such a view, the 

 " remote centers of differentiation " could be 

 dispensed with. 



If, as Grinnell believes, the two sets of 

 animals " have undoubtedly descended from 

 ancestral lines, which have invaded the terri- 

 tory from the two opposite directions," already 

 specifically distinct, we can not see the force 

 of the conclusion that " adequate ground 

 is afforded for the belief that intervention of 

 barriers is a prime factor in the differentiation 

 of species." All the evidence shows in this 

 case is that the barrier has kept these species 

 apart. It may have had nothing to do with 

 their differentiation as species. Indeed, in the 

 absence of experimental evidence, we can not 

 even affirm with certainty that any physical 

 barrier has been necessary for the continued 

 maintenance of their specific distinctness. It 

 is not impossible that a high degree of sterility 

 would be found to exist between the California 

 and the Arizona species. Nevertheless, the 

 facts, as described, are of great interest. Fur- 

 ther expeditions should be sent into the valley 

 of the Colorado for the express purpose of test- 

 ing some of these important questions. And 

 the work should be done before nature's orig- 

 inal scheme of distribution has become hope- 

 lessly muddled through man's agency. 



Grinnell recognizes " three distinct orders 

 of distributional behavior as regards terrestrial 

 vertebrates." First, 



every animal is believed to be limited in distribu- 

 tion eonally by greater or less degree of tempera- 

 ture, more particularly by that of the reproduc- 

 tive season. . . . When a number of animals (al- 



ways in company with many plants similarly re- 

 stricted) approximately agree in such limitation, 

 they are said to occupy the same life-zone. 



Throughout this and many other papers by 

 the same author the " life-zone " conception 

 plays a prominent role. The zones recognized 

 by C. Hart Merriam are adopted by Grinnell, 

 and their existence accepted as a fundamental 

 datum, without the necessity of their being 

 justified to the reader. The author believes, 

 following Merriam, that the position and ex- 

 tent of these " zones " is determined by tem- 

 perature conditions. Yet it is obvious that, 

 throughout considerable portions of the con- 

 tinent, the details of temperature distribution 

 are not known with any approach to precision. 

 Thus, the actual criterion which the field 

 zoologist falls back upon in any given case is 

 the character of the fauna and flora which he 

 finds associated together. The presence of 

 certain species shows him that he chances to 

 be in this or that " life-zone." It is assumed, 

 though apparently seldom verified, that wher- 

 ever these particular species occur in conjunc- 

 tion, the temperature conditions are in some 

 essential respect similar. 



It would seem a priori that in traveling 

 along a uniform gradient from a region of 

 higher to one of lower average temperature, 

 or vice-versa, one would continually pass into 

 and out of the ranges of species which found 

 their limits of physiological adaptability at 

 different points along the line. One would 

 scarcely expect to encounter critical points, 

 where the fauna and flora as a whole, or at 

 least the most characteristic members of it, 

 were suddenly replaced by a quite different 

 assemblage. Yet this is the essence of the 

 " life-zone " conception. 



It would be foolhardy, indeed, for a zoologist 

 of limited field experience to criticize this con- 

 ception. It is doubtless based upon extensive 

 and accurate observations and represents real 

 facts. But unfortunately they are, in a high 

 degree, facts which, by their very nature, are 

 scarcely communicable to most biologists. Be- 

 fore the life-zone conception can be of much 

 service to the average student of evolutionary 

 problems it will have to be expressed in terms 



