444 DIRECTIVE FACTORS IN EVOLUTION: 



be convenient to discuss some of the positive changes in the 

 theory. 



(1) There has been in a few cases a welcome demonstra- 

 tion of Natural Selection at work. The theory is not merely 

 a hypothesis as to what might have happened long ago; it 

 is a statement of what does happen now. There is some 

 actual proof of discriminate selection, where the survivors 

 are shown to survive in virtue of the possession of particular 

 qualities. Let us take a well-known diagrammatic instance, 

 (Cesnola, Biometrika, 1904). The praying Mantis, Mantis 

 religiosa, occurs in Italy in a green and a brown variety, 

 the former usually on the grass, the latter usually on the 

 withered herbage. The Italian naturalist Cesnola tethered 

 twenty green Mantises among green herbage and a similar 

 number of brown ones among withered grass. After seven- 

 teen days they were all alive, having escaped the notice 

 of their enemies. He tethered twenty-five of the green 

 variety among brown herbage ; all were dead after eleven 

 days. In the converse experiment, of forty-five brown insects 

 exposed on green grass, only ten survived at the end of sev- 

 enteen days. Most of the Mantises were killed by birds ; 

 five of the green ones were killed by ants. The experiment 

 should be extended, but it proved the selective value of 

 the coloration. If green and brown Mantises were exposed 

 in a green country, the green ones would survive, the brown 

 ones would be eliminated, and the selective death-rate would 

 have reference to the particular quality of coloration. Simi- 

 lar experiments have been recorded by Professors Poulton, 

 Crampton, Bumpus, and Weldon — all proving discriminate 

 elimination. Prof. Karl Pearson has also demonstrated the 

 occurrence of a selective death-rate in man. 



These demonstrations require more exposition than is here 



