Febbtjabt 23, 1906.] 



SCIENCE. 



311 



no disagreement. Following President Jor- 

 dan's original statement of the case in the 

 issue of Science for November 3, 1905, and 

 my own comment thereon in the issue for 

 November 24, 1905, is a communication by 

 Professor E. A. Ortmann, in the issue for 

 January 12, 1906, entitled 'Isolation as One 

 of the Factors in Evolution,' about which I 

 ■wish to offer a few words in the way of 

 comment. 



In this communication Professor Ortmann 

 states, apropos of the previous papers by Jor- 

 dan and myself, that he ' can not strongly 

 enough endorse ' Jordan's view ' that isolation 

 is a factor in the formation of every species 

 on the face of the earth ' ; ' for,' he continues, 

 ' it is absolutely unthinkable that two species 

 may be derived from one ancestral species 

 without the action of isolation.' To continue 

 the quotation further, Ortmann says : 



All the instances introduced by Allen as op- 

 posed 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 distribution. Indeed, there is con- 

 tinuous distribution, but there is no continuity 

 of bionomic conditions. These different bionomic 

 conditions 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. 



Under the new definitions of ' barriers ' and 

 ' isolation ' this may, in large part, be con- 

 ceded as true, but as not wholly true, even 

 under these new premises. If President Jor- 

 dan originally meant by isolation and barriers 

 * bionomic isolation ' and ' bionomic barriers,' 

 as he has since stated that he did,' and as 

 Professor Ortmann now claims that he did, 

 instead of isolation and barriers as commonly 

 accepted by students of the geographic distri- 

 bution and geographic evolution of animals, 

 the case is, of course, quite changed by the 

 afterthought of fuller definition. It may, 

 further, explain Ortmann's statement that my 

 presentation of the case ' demonstrates again 



• Science, N. S., Vol. XXII., No. 570, p. 715. 



that the principle of isolation or separation 

 is not generally understood in its full mean- 

 ing.' 



But let us consider for a moment just what 

 are the real facts covered by the statement: 

 ' Indeed, there is continuous distribution, but 

 there is no continuity of biologic conditions' 



Students of the geographic distribution and 

 geographic or climatic evolution of species 

 and subspecies, and also of minor local vari- 

 ants, among North American birds and mam- 

 mals, both in the field and through handling 

 vast numbers of museum specimens, are, per- 

 haps, as familiar with the general facts of 

 geographic variation as are investigators in 

 any other field of biology. Let us apply a 

 little of this ' common knowledge ' to the 

 statement that " for complete isolation ' bar- 

 riers ' in most cases are not absolutely neces- 

 sary features." We may take for illustration 

 any one of hundreds of conspecific groups, 

 such as the song sparrows, the quails, grouse, 

 woodpeckers, ground squirrels of several gen- 

 era, tree squirrels, hares, field mice of various 

 genera, etc., where an intergrading group of 

 well-marked geographic forms has collectively 

 a continuous range of hundreds, and often of 

 several thousand miles in, it may be, both 

 an east and west and a north and south direc- 

 tion. The extremes, or the peripheral forms, 

 are so diverse in size, coloration, food habits, 

 etc., that if one of these extreme forms were 

 to be transferred to the home of the other it 

 would be impossible for the two to intermingle 

 through interbreeding, or for either to survive 

 the changed conditions of environment. Yet 

 between them there is no impassable physical 

 barrier to continuous distribution, nor any 

 break in the continuity of intergradation. 

 Between such forms there is evidently a hio- 

 nomic harrier. They have, indeed, become so 

 divergent that were the connectant forms 

 swept out of existence over a wide area by an 

 epidemic of disease, or by some topographic 

 or climatic change of conditions, the surviving 

 extremes of the series could be considered by 

 systematists in no other light than as fully 

 segregated and sharply defined species. 



But how is it between two contiguous and 

 less differentiated forms? In eastern North 



