Januaet 12, 1906.] 



SCIENCE. 



71 



a calcareous soil, viz., one that will ' effervesce 

 witli acids ' (requiring the presence of at least 

 three per cent, of carbonate), goes far beyond 

 what insures the presence of calciphile plants 

 in thousands of cases. I have elsewhere 

 summed up what may be said on this point, 

 to the effect that while in heavy clay lands 

 as much as six tenths per cent, of lime in the 

 soil may be necessary to secure the advantages 

 of calcareous lands, in the ease of light sandy 

 soils one tenth per cent, may be sufficient to 

 produce natural calciphile growth, and, there- 

 fore, also the cultures which, like the legumes, 

 demand soils which are not only neutral, but 

 which shall be able to supply to them freely 

 the lime which forms so prominent an ash 

 ingredient. 



In this, the proper sense of the word, cal- 

 careous soils will be found to exist not only in 

 limestone districts, but in all derived from 

 hornblendic rocks, including black lavas and 

 basalts, and also from the rocks containing 

 either labradorite or some of the soda-lime 

 feldspars. Such soils rarely effervesce, but 

 when wetted they show with red litmus paper, 

 at the end of twenty minutes, the blue reac- 

 tion which is wholly independent of ' alkali.' 

 Even dilute acetic acid will in that case 

 readily dissolve from the soil enough lime to 

 give a plain reaction with oxalates. 



I trust that this point of view may be made 

 the subject of verification by Mr. Wilder as 

 well as others. E. W. Hilgakd. 



Berkelet, Cal., 

 December 8, 1905. 



ISOLATION AS ONE OF THE FACTORS IN EVOLUTION. 



It was with much pleasure that I read the 

 article of President D. S. Jordan on ' Isola- 

 tion ' in a recent number of Science,' and, 

 aside from the fact that I am able to add a 

 large number of cases, I have nothing to com- 

 ment upon. But the subsequent article by Pro- 

 fessor J. A. Allen^ demonstrates again that 

 the principle of isolation or separation is not 

 generally understood in its full meaning. 



Jordan expresses the opinion that isolation 

 is a factor in the formation of every species 

 on the face of the earth. I can not strongly 



' Science, November 3, 1905, p. 545 if. 



^Science, November 24, 1905, p. 661 ff. 



enough endorse this view, for it is absolutely 

 unthinkable that two species may be derived 

 from one ancestral species without the action 

 of isolation. All the instances introduced by 

 Allen as opposed to this view are rather in 

 support of it. He concludes that in variations 

 of certain widely distributed species, which 

 pass into each other from one extremity of 

 the range to the other, no isolation by barriers 

 exists, but that there is continuous distribu- 

 tion. Indeed, there is continuous disiribu- 

 tion, but there is no continuity of hionomic 

 conditions. These different bionomic condi- 

 tions pass into each other, and, consequently, 

 we have varieties, and not species. This is 

 clearly the first step toward complete isolation, 

 and for complete isolation ' barriers ' in most 

 cases are not absolutely necessary features. 



It is not quite correct to conceive isolation 

 only iu its coarsest sense, as topographic or 

 climatic separation. This mistake is often 

 made, but I pointed out, about ten years 

 ago, that the real and most important value 

 of the principle of separation lies in its gen- 

 eral hionomic sense. The same idea was 

 maintained long ago by Gulick, and has been 

 treated recently by him in an elaborate mono- 

 graph.^ I am fully in accord with most of 

 Gulick's ideas as to the influence of separa- 

 tion upon the formation of species, chiefly as 

 opposed to the senseless abuse of the term 

 species introduced by the de Vries school. 

 ' Bionomic separation,' as used by myself, and 

 ' habitudinal segregation,' as used by Gulick, 

 are practically identical terms. 



With Jordan (and with Gulick) I believe 

 that ' bionomic separation ' is absolutely neces- 

 sary for the formation of species, but that it 

 is not the only factor taking part in the 

 process called ' evolution.' With regard to 

 this, I may be permitted to quote from a paper 

 published by myself in 1896,' which seems to 

 have been overlooked generally : 



* * * ^e have to distinguish four factors ac- 

 complishing the diversity, development and dif- 

 ferentiation into species of organic beings : we 



= Gulick, J. T., 'Evolution, Racial and Habi- 

 tudinal,' Carnegie Institution, Washington, 1905. 



* ' On Natural Selection and Separation,' Pr. 

 Amer. Philos. Sac, 35, 1896, pp. 175-197, espe- 

 cially pp. 188-190. 



