210 ENVIRONMENTAL CONDITIONS. 



Merriam's wory and that of his colleagues of the Bureau of Biologi- 

 cal Survey of the U. S. Department of Agriculture constitute by far 

 the most thorough study that has yet been brought forth of the rela- 

 tions of plant and animal distribution to temperature conditions in the 

 United States, and Merriam's temperature integrations have furnished, 

 for two decades, practically the only available information in this 

 regard. In his most complete account of this arduous work of integra- 

 tion, Merriam writes: 



Several years ago I endeavored to show that the distribution of terrestrial animals and 

 plants is governed by the temperature of the period of growth and reproductive activity, 

 not by the temperature of the whole year; but how to measure the temperature concerned 

 was not then worked out. * * * At one time I beheved that the mean temperature of 

 the actual period of reproductive activity in each locaUty was the factor needed, but such 

 means are almost impossible to obtain, and subsequent study has convinced me that the 

 real temperature control may be better expressed by other data. * * * If it is true that 

 the same stage of vegetation is attained in different years when the sum of the mean daily 

 temperature reaches the same value, it is obvious that the physiological constant of a species 

 rmist be the total quantity of heat or sum of positive temperatures required by that species to 

 complete its cycle of development and reproduction. * * * I am not aware that an attempt 

 has been made to correlate the facts thus obtained with the boundaries of the life zone. * * * 

 If the computation can be transferred from the species to the zone it inhabits — if a zone 

 constant can be substituted for a species constant — the problem will be well nigh solved. 

 This I have attempted to do. In conformity with the usage of botanists, a minimum 

 temperature of 6° C. (43° F. [42.8° F., see footnote, p. 231])^ has been assumed as marking 

 the inception of the period of physiological activity in plants and of reproductive activity 

 in animals. The effective temperature or degrees of normal mean daily heat in excess 

 of this minimum has been added together for each station, beginning when the normal 

 mean daily temperature rises higher than 6° C. in spring and continuing until it falls to 

 the same point at the end of the season. The sums thus obtained have been platted on a 

 large scale map of the United States, and isotherms have been run which are found to con- 

 form in a most gratifying manner to the northern boundaries of the several life zones. * * * 

 While the available data are not so numerous as might be desired, the stations in many 



^ Merriam (1894.) A very much abbreviated statement of the results embodied in the above 

 paper was published as Part III of the same author's Life Zones and Crop Zones of the United 

 States, U. S. Dept. Agric, Div. Biol. Survey. Bull. 10, 1898. This latter involves but two pages 

 (54, 5.5) and does not include the climatic map. Nothing approaching an adequate presentation of 

 the data upon which these important studies are based has, as far as we are aware, ever appeared. 



It can not be too strongly emphasized that work of this sort is deprived of by far the greater 

 part of its possible usefulness in building up our knowledge whenever a derived chart is published 

 without the station data upon which it is based. It appears that most writers who have dealt 

 with climatic charts have considered these as an end rather than as a means. Such charts are 

 simply broad and necessarily very general presentations of the facts or observations upon which 

 they are constructed, and can accomplish little more for the student of plant distribution or of 

 agriculture than to inform him where in the given region to look for stations with certain climatic 

 characteristics. As soon, however, as his interest is thus aroused he requires the station data, 

 and if these are not at hand, further quantitative studies therewith are effectually precluded. We 

 will not suppose that this common suppression of basic data is to be related at all to any desire 

 on the part of writers to veil the exact methods of their procedure in the preparation of charts; 

 we suppose rather that the suppression here in view has usually arisen from lack of facilities for 

 publication or from lack of time and energy requisite for the preparation of tables or for the 

 placing of the numerical data upon the published charts. It is hardly conceivable that a writer 

 who has derived important generalization from a mass of figures should not appreciate the 

 probability that the same figures may be utilized, in the same or in different ways, by other 

 students of the subject. 



Had Merriam's publications included a list of stations each with its numerical climatic indices, 

 the latter might have been put to many other u.ses than the mere preparation of the simple charts. 



* As the chart was published, however, the minimum here referred to was 0° C. (32° F.). See 

 Merriam's note, Science, n.s., 9: 116, 1899. 



