8 Trans. Acad. Sci. of St. Louis. 



November, but exceptionally the last of September even in the 

 southeast. The last frost occurs in the south about the first, and 

 m the north about the fifteenth of April, exceptionally later as on 

 April 20, 1904, when six inches of snow covered the ground at 

 St. Louis with a temperature of 28° to 30° (max.). Hoarfrosts 

 may kill tender vegetation as late as the middle of May in nearly 

 all pa]ts of the state. 



The following dates may illustrate the remarkable dissimilarity 

 in dates of opening spring: Peach-trees were in bloom in St. 

 Louis in 1878 on March 15; in 1907 on March 25; in 1879 on April 

 15 ; in 1880 on April 1 and in 1881 on April 28. The same Magnolia 

 which was in flower on March 12, 1878, did not bloom in 1881 

 before April 24, but in 1882 again as early as March 18, when 

 spring opened on the first of March. Though spring opened in 

 1881 only on April 16 not a single tree was without its leaves at 

 St. Louis on May 9; but in 1907 the leafing of trees began March 

 15 and was not completed June 1. An exceptionally early open- 

 ing of spring with us can, of course, have no influence on the 

 starting of migrants from their remote winter homes in southern 

 Mexico, Central and South America, as they cannot know what 

 kind of weather we have in the United States, but a late spring 

 may retard their progress after they have entered our country. 

 Most of the birds which winter beyond the limits of the United 

 States do not reach Missouri before April, and their arrival is there- 

 fore not influenced by our weather prior to that time. They do 

 not come earlier, be the spring ever so early and vegetation corres- 

 pondingly advanced; but it is different with birds which winter 

 wHhin the United Stales, as nearly all species do which arrive in 

 Missouri prior to A])ril. Though the desire to return to their 

 breeding ground is not dependent on the weather, being the result 

 of a physiological process which through inheritance is fixed to a 

 certain time of the year independent of meterological conditions, 

 a precocious rise in temperature with the consequent develop- 

 ment of plant and animal life exerts some influence by stimulat- 

 ing this desire, and it is for this reason that considerabl'^ fluc- 

 tuation occurs in the tim.e of arrival of our earlier migrants as 

 well as in the depaiture of our winter guests. A backward spring 

 causes a general retardation of all migration that becomes less 

 marked as the season advances, but every cold wave, even in 

 the height of migration, checks farther advancement for the time 

 being and detains transients at the localities where they happen 

 to be when the adverse conditions arise. This is of great prac- 



