CORRELATION OF DISTRIBUTIONAL FEATURES. 567 



than for the western species, but are comprised in the wide amplitudes 

 of the latter. The amphtudes of the rainy periods and the dry periods, 

 on the other hand, are such as to indicate that the two species have 

 almost nothing in common with respect to these conditions. 



The occurrence of Trautvetteria grandis over both interior and coastal 

 regions in the Northwest is responsible for wide amplitudes of evapora- 

 tion, humidity, and moisture ratios, for all of which the amplitudes are 

 narrow for the eastern species. Climatological data from such locali- 

 ties as Helena, Boise, Walla Walla, and Spokane are not suited, 

 however, to giving an accurate conception of the conditions for this 

 plant. The moisture conditions of those stations are doubtless much 

 more severe than those of the southernmost mountain localities for 

 Trautvetteria grandis in New Mexico. Even though our graphs may 

 indicate conditions of evaporation that are too high for Trautvetteria 

 grandis and conditions of humidity and moisture ratio that are too 

 low, there remains, nevertheless, a marked difference between the 

 amplitudes of these conditions for the eastern and western species, 

 since the low range of evaporation and the high range of humidity and 

 moisture ratios are the conditions in which the western species is most 

 abundant. In this case our climatological stations are located in the 

 midst of the conditions in which it actually grows. 



The differences in the extremes and amplitudes of the principal 

 climatic conditions for the two species of Trautvetteria are sufficiently 

 great to indicate that it would be difficult to grow either of them 

 throughout the range of the other. The distinctive conditions under 

 which the two species now grow, together with their complete geo- 

 graphical segregation, must be taken to mean that they are neither 

 recent nor immediate derivatives from a common ancestral stock. 



Populus balsamifera and Sapindus marginatus (fig. 73). — A ready 

 comparison of climatic extremes for these two trees has been made 

 possible by placing the blocks for the two on the same diagram. Sapin- 

 dus is a tree of southern range, extending from central Texas and 

 southern Kansas to Florida. Populus is a tree of northern range, 

 extending from Connecticut to North Dakota, and through Canada to 

 the northern Rocky Mountains, where it occurs in a form which has 

 been recently regarded as a distinct species (see plate 19). 



Since the distributional areas of these two trees are quite separate 

 and yet are nowhere less than 400 miles apart, it is not surprising that 

 the ranges of temperature conditions are so unlike as to overlap only 

 in the case of the length of the frostless season, where the maximum for 

 Populus is slightly in excess of the minimum for Sapindus. The pre- 

 cipitation conditions for these two trees are such that there is a con- 

 siderable range of precipitation values common to both of them, 

 although the extremes are by no means the same. The amphtudes of 

 evaporation and humidity are much greater for Populus than for 



