December 29, 1905.] 



SCIENCE. 



873 



An ' ontogenetic species ' its traits produced 

 by the direct action of the environment, is 

 the Loch Leven trout C Salmo levenensis'), 

 which I have lately discussed in these col- 

 umns. Transferred to the brooks of England 

 or to those of California, this supposed species 

 loses its lake-bred characters and becomes the 

 common brook trout. 



Perhaps our ornithologists vpill some day 

 test their species and subspecies by a test of 

 the permanence of this class of characters. 

 No doubt we should drop from the systematic 

 lists all forms which may prove to be purely 

 ontogenetic, all whose traits are not fixed in 

 heredity. 



In' my recent article, as noticed by Dr. 

 Allen, I have used the word ' barrier ' a little 

 too vaguely. For the purposes of this study, 

 I should regard a broad plain as a barrier to 

 a species which inhabits it, even though it 

 were abundant, from one side to the other. 

 A barrier in this sense is anything whatever 

 which checks free interbreeding, even though 

 it oilers no actual check to the life or move- 

 ment of the species. With quiescent animals, 

 the individual moves but a short distance and 

 the traits at one end of an unbroken series 

 may be quite different from those of other 

 individuals at the farther end, as Dr. Allen 

 has very properly su.ggested. The term ' bio- 

 nomic barrier,' used by Dr. A. E. Ortmann 

 in a personal letter, seems to me a very apt 

 one as covering the species-producing phases 

 of isolation. 



Certain paioers of Rev. John T. Gulick on 

 the evolution of species of land-snails and 

 other animals deserve more attention than 

 they have received. In one of these papers, 

 ' Divergent Evolution through Cumulative 

 Segregation ' (Smithsonian Report for 1891, 

 p. 273), Mr. Gulick corrects certain erroneous 

 assumptions on the part of Dr. Moritz Wag- 

 ner. Mr. Gulick says: 



Separate generation is a necessary condition for 

 divergent evolution but not for the transformation 

 of all the survivors of a species in one way. 



Separation does not necessarily imply any ex- 

 ternal barriers or even the occupation of separate 

 districts. 



Diversity of natural selection is not necessary 

 to diversity of evolution. 



Difference of external conditions is not necessary 

 to diversity of evolution. 



Separation and variation — that is, variation not 

 overwhelmed by crossing — is all that is necessary 

 to secure divergence of type in the descendants of 

 one stock, though external conditions remain the 

 same and tliough the separation is other than 

 geological. The separation I speak of is anything 

 in the species or the environment that divides the 

 species into two or more sections that do not 

 freely intercross, whether the different sections 

 remain in the original home or enter new and dis- 

 similar environments. 



All of this is in general accord with my own 



experience. 



David Starr Jordan. 



ORTHOGENETIO VARIATION ? 



In a recent paper^ I reviewed Gadow's hy- 

 pothesis of ' Orthogenetic Variation,' ^ in the 

 light of his own evidence, and in the light of 

 such observations as could be added. In 

 Science for November 7, 1905, Dr. Gadow 

 publishes a reply under the title ' Orthogenetic 

 Variation.' It would be superfluous merely 

 to rediscuss the data previously published; in 

 fact, had the matter gone no further than the 

 original paper, elaborate criticism in the first 

 instance might have been unnecessary, since 

 scientific readers could judge the evidence for 

 themselves. But unfortunately, as will be 

 shown below, subsequent use has been made 

 of the idea for presentation to the general 

 public, not expressly as a tentative hypothesis 

 — but without qualification. 



In the first paragraph of his reply, Gadow 

 says, ' I am anxious that it [orthogenetic 

 variation] should not be misrepresented,' and, 

 in the second paragraph, ' the paper by Mr. 

 Robert E. Coker - * - calls for some remarks 

 on my part by way of protest and correction.' 

 I was glad that after careful reading of his 

 paper, I found no reference to any statement 



^ Gadow's hypothesis of ' Orthogenetic Variation 

 in the shells of Chelonia,' Johns Hopkins Univer- 

 sity Circular, No. 178, May, 1905. 



^ ' Zoological Results Based on Material from 

 New Britain, New Guinea, Loyalty Islands and else- 

 where, collected during the years 1895, 1896 and 

 1897, by Arthur Willey,' Part III., pp. 207-222, 

 PI. XXIV., XXV., Camb. Univ. Press, May, 1899. 



