furthermore, the period of scientific and intense forestry 

 and of economic entomology. Here we have the probable 

 agencies that made life difficult for the moth and gave a 

 great selective value to that advance in hardiness which 

 lies behind the melanic appearance. 



We should point out here the difficulties which arise in 

 the criticism of definite views of evolution on the basis of 

 facts not analyzed genetically. The selective value of a 

 climatic character may often be doubted on the ground 

 that the' same type occurs in a very different area ad- 

 mixed with the local form. But genetic analysis may 

 often show that what appears to be the same type is in 

 reality a different thing. The north European Arctiid, 

 CaUimorpha domhuda, has wings marked with red; the 

 Italian form has wings marked with yellow. In certain 

 localities (one of them near Berlin) a yellow sport of the 

 red form regularly appears, apparently the same form as 

 the Italian one. We, as well as others, have crossed these 

 forms. The yellow sport is a simple recessive to red and 

 segregation occurs in the 3 : 1 ratio. The Italian yellow 

 form, however— at least the ones from the Abruzzi, which 

 we used— crossed with the red northern form, produces 

 intermediate orange in F 1 and in F 2 every shade from red 

 to yellow. The two yellows, which look alike and prob- 

 ably are chemically alike, are nevertheless products of a 

 different metabolic process. In the sport the same met- 

 abolic process which usually leads to red pigment is 

 changed by mutation only to the extent of the color change 

 in the end-product. In the southern form a different type 

 of metabolism results in the formation of yellow pigment, 

 and the cross is therefore an entirely different cross, with 

 different results. 3 As a matter of fact, the latter cross 

 shows very much diminished fertility, as Standfuss has 

 already pointed out. This shows how unsafe the ground 

 is on which criticism of evolutionary questions without 

 genetic test is based. That our example is not an excep- 

 tion is proved by the fact that Standfuss long ago formu- 



