474 R. T. YOUNG 



Experiment 7. Figure o. In this experiment two Mus were placed 

 venter iipperaiost on dark earth and two on a mixtin;e of dead leaves 

 of the Cottonwood tree (Populus) and the staminate branches and the 

 styles of corn (Zea). As viewed from the opposite end of the cage, 

 the resemblance between the mice and this latter background was 

 close, while the mouse-earth combination presented a strong contrast. 

 In six minutes a hawk flew direct to the backgrounds and picked a 

 mouse from the earth. 



Experiment 8. The arrangement of this experiment was the same 

 as Experiment 7, except that only one mouse was placed on each back- 

 ground. In one minute a hawk dropped to the ground from its perch 

 and ran to a point near the backgrounds, but did not feed. One min- 

 ute later the other flew from its perch direct to the backgrounds and 

 took the mouse on the earth. 



Experiment 9. The arrangement here was the same as in Experi- 

 ment 8, except that the backgrounds were reversed in position, and 

 the venter of the mouse on the earth was brought flush with the earth's 

 surface. It was twilight when the experiment was started (5.54 p.m., 

 September 28). In nine minutes a hawk flew from its perch directly 

 over the mice but apparently did not notice them. In twenty-c^ne 

 minutes the other flew direct to the backgrounds and took the mouse 

 on the earth. 



Experiment 10. Figure 3. Two Microtus were employed in this 

 experiment, one being placed on ashes and one on a background of 

 dark earth over which were scattered pieces of bark and dead leaves. 

 The mouse on the latter background was plainly visible to my eye 

 from the opposite end of the cage, but the contrast between the mouse 

 and the ash background was much plainer. After looking at the 

 mice for several seconds, a hawk flew direct to the backgrounds and 

 took the mouse from the ashes. 



Experiment 11. This experiment was the same as the last but the 

 position of the backgrounds was reversed. In seven minutes a hawk 

 after looking at the backgrounds flew to within several centimeters of 

 them, paused a moment, and then took the mouse from the ashes. 



Experiment 12. Figure 3. In this experiment two Peromyscus 

 were placed venter uppermost one on ashes mixed with a little earth 

 and one on earth. The venter of each mouse was brought flush with 

 the surface of the background. As seen from the opposite end of the 

 cage, the resemblance between the mouse and the ashes was so close 

 • as to render it invisible to my eye. In eight minutes a hawk flew to 

 the ground at the middle of the cage, and walking slowly to the back- 

 grounds took the mouse from the earth. Shortly afterward the other 

 hawk passed very near the other mouse, but did not notice it. Five 

 minutes later, however, the former hawk took it. 



Experiment 13. Figure 16. A smaller Mus was placed on ashes 

 and a larger one on earth mixed with a little ashes and partly covered 

 with bits of bark, dead leaves and straw. The resemblance here be- 

 tween the mouse and th(> latter background was close. In twenty- 



