34 



BULLETIN 185, U. S. DEPARTMENT OF AGRICULTURE. 



which were very slight, whether over land or water. From this 

 short migration benefit accrued to individuals or to their posterity. 

 Migration became a fixed habit, and the distance covered gradually — 

 very gradually — increased as each succeeding extension proved 

 advantageous. It is not to be supposed that every attempted 



Fig. 15.— Summer and winter homes of the black-and-white warbler ( Mniotilta varia). A very slow mi- 

 grant. The isochronal lines of migration indicating the rate of speed are shown in figure 16. (See p. 25.) 



extension was successful; in fact, it is more probable that only a 

 small part of the experimental pioneer routes were permanently 

 adopted. Moreover, it must be borne in mind that the time occu- 

 pied in the establishment of present migration habits and routes 

 is to be measured by geologic ages, and there is no reason to suppose 

 that changes took place during these ages any faster than they do 



