266 EVOLUTION, GENETICS, AND EUGENICS 



placed a number of crabs in a large aquarium, in which china-clay was 

 kept partly in suspension, and found that about half of them died. 

 Again the survivors were compared statistically with the perished and 

 the same relation was found to hold: that the survivors had a mean 

 frontal breadth distinctly narrower than that of the perished. Wel- 

 don concludes that his experiments "have demonstrated two facts 

 about these crabs; the first that their mean frontal breadth is dimin- 

 ishing year by year at a measurable rate, which is more rapid in males 

 than in females; the second is that this diminution in frontal breadth 

 occurs in the presence of a material, namely, fine mud, which is 

 increasing in amount, and which can be shown experimentally to 

 destroy broad-fronted crabs at a greater rate than crabs with narrower 

 frontal margins .... and I see no escape from the conclusion 

 that we have here a case of Natural Selection acting with great 

 rapidity, because of the rapidity with which the conditions of life 

 are changing." 



Cesnola's experiments with Mantis. — To test the selective value 

 of color markings Cesnola fixed specimens of the brown and green 

 Mantis religiosa on plants, some of which were against harmonious, 

 others against disharmonious backgrounds. The result was that most 

 of those which were inconspicuous because of a harmonious back- 

 ground escaped, while most of the others were eaten up by birds. 



Poulton's and Sanders' experiments with butterfly pupae. — 

 Numerous pupae of various colors were placed under conditions favor- 

 ing protective coloration and others under opposite conditions. The 

 conclusion was that protective coloration is a real survival factor, and 

 one that operates so as to give the protectively colored individual 

 a decided advantage in the struggle for existence. 



Davenport's experiments with chickens. — A number of chickens, 

 some black, some white, and some barred or checkered in color, were 

 allowed to wander free in the fields. Hawks killed most of the whites 

 and many of the blacks, but spared, to a large extent, the less con- 

 spicuous checkered and barred types which are harder to detect 

 against a mixed background. 



All of these experiments merely tend to show that discriminate 

 survival actually occurs, but only the experiment of Weldon has a 

 bearing on the possibility that mere quantitative changes of small 

 dimensions might under certain conditions be of selective value. We 

 badly need more experimental evidence of this sort and until this 

 is forthcoming; we shall have to admit that there is very little 



