558 



SCIENCE. 



[N. S. Vol. XXIV. No. 618. 



intergrades are possible. If, on the contrary, 

 the characteristic refuses to blend, but comes 

 out of the cross intact, as it went in, the con- 

 clusion seems justified that the characteristic 

 is essentially integral and must have arisen 

 completely formed, and hence discontinu- 

 ously. 



Using this criterion, I have of late been 

 testing the application of the mutation theory 

 to animals and have had an opportunity to 

 examine the experiments of others. Some of 

 the work has been done on the characteristics 

 of domesticated 'races,' others on wild varie- 

 ties. There seems to be no difference in the 

 behavior of characteristics of domesticated 

 and wild varieties. The result is that most 

 characteristics, but not all, fail to blend and 

 are strictly alternative in inheritance. I in- 

 terpret this to mean that the characteristic 

 depends on a certain molecular condition that 

 does not fractionize. The inference is that 

 if the characteristic is incapable of gradations 

 now it has always been and hence must have 

 arisen without gradations, i. e., discontinu- 

 ously. Examples of such discontinuous char- 

 acteristics are the spots on the elytra of cer- 

 tain beetles, the crest on the canary, the form 

 of the comb in poultry, extra toes, black 

 plumage and color of iris. One who sees the 

 striking failure of these characteristics and 

 many others to be modified in any important 

 way will feel convinced that they are not 

 capable of forming intergrades and hence 

 could not have arisen gradually. 



While I am not of those who would seem to 

 deny that characters of domesticated species 

 are as natural as any others, it is worth in- 

 quiring whethej- discontinuous variations, 

 such as I have been dealing with, occur among 

 feral animals. The evidence is that they do. 

 Thus our gray squirrels exhibit in many 

 localities a striking number of black indi- 

 viduals. These are not found everywhere, 

 but in small areas may be fairly common. 

 Difference in climatic conditions can not ac- 

 count for the blacks — they belong to the order 

 of melanic sports, *. e., mutations. Our red 

 squirrels and various other feral rodents sport 

 in the same way. Birds also show melanic 

 sports, e. g., in the European snipe {Scolopox 



gallinago) a chocolate brown form sometimes 

 appears which, like the black squirrel, has been 

 considered by some as a distinct species. 

 Similarly, more or less albinic sports occur in 

 nature. White crows and blackbirds are well 

 known and many individuals of the house 

 sparrow are partially albinic. The history of 

 the twisted beak of the crossbill (Loxia curvi- 

 rostra) is, of course, unknown. The char- 

 acteristic is, however, the same as, and has 

 probably had a similar origin with, that which 

 suddenly appears in one per cent, of the poul- 

 try that I breed and, which has been observed 

 as a sport in crows. Scarcely one of the 

 characteristics of poultry may not be found 

 appertaining to some feral species, and there 

 is every reason to believe that these character- 

 istics have the same property of indivisibility 

 in the latter case as in the former. Such 

 facts as I have cited above could be added to 

 by Dr. Merriam or any other naturalist with a 

 similarly extensive and profound knowledge 

 of the higher vertebrates; and they seem to 

 me to lead to the conclusion that some new 

 characters may arise in nature suddenly, as 

 sports or mutations, and persist as specific 

 characteristics. 



Chas. B. Davenport. 

 Station fob Experimentaj: Evolution, 

 October 18, 1906. 



THE RIGIDITY OF THE EARTH. 



To THE Editor of Science: In Science of 

 September 28 Professor L. M. Hoskins is led 

 to the sad conclusion that I have misunder- 

 stood Lord Kelvin's definition of the modulus 

 of rigidity, and he thus apparently questions 

 the results which I have given in Astronom- 

 ische NachricMen, No. 4,104. Owing to the 

 great length of that paper, my explanation of 

 the connection between rigidity as experimen- 

 tally determined for solid bodies here upon the 

 earth's surface and other bodies kept rigid by 

 pressure was not sufficiently developed; and 

 as the difficulty that has misled Professor 

 Hoskins appears to have occurred also to 

 others, it seems worth while to point out the 

 omitted steps in the chain of reasoning, whi-ch 

 will, I think, make it clear that my process 

 has been misunderstood and misinterpreted. 



