662 



SCIENCE. 



[N. S. Vol. XXII. No. 5G9. 



question or controvert any o"f the evidence, 

 or in any way to belittle the principle of 

 evolution by isolation, but to bring forward 

 other well-known facts that bear on the 

 problem of evolution through environment 

 unassisted by isolation or obvious physical 

 barriers. 



Says President Jordan, in his opening 

 paragraph : 



It is now nearly forty years since Moritz Wag- 

 ner (1868) first made it clear that geographical 

 isolation (rjiumliche Sonderung) was a factor or 

 condition in the formation of every species, race 

 or tribe of animal or plant we know on the face 

 of the earth. This conclusion is accepted as almost 

 self-evident by every competent student of species 

 or of the geographical distribution of species. 

 But to those who approach the subject of evolution 

 from some other side the principles set forth by 

 Wagner seem less clear. They have never been 

 confuted, scarcely even attacked, so far as the 

 present writer remembers, but in the literature of 

 evolution of the present day they have been almost 

 universally ignored. Nowadays much of our dis- 

 cussion turns on the question of whether or not 

 minvite favorable A'ariations would enable their 

 possessors little by little to gain on the parent 

 stock, so that a new species would be established 

 side by side with the old, or on whether a wide 

 fluctuation or mutation would give rise to a new 

 species which would hold its own in competition 

 with its parent. In theory, either of these condi- 

 tions might exist. In fact, both of them are 

 virtually unknown. In nature a closely related 

 distinct species is not often quite side by side 

 with the old. It is simply next to it, geograph- 

 ically or geologically speaking, and the degree of 

 distinction almost always bears a relation to the 

 importance or the permanence of the barrier 

 separating the supposed new stock from the parent 

 stock. 



With the above I am in hearty accord, 

 except with the declaration that geograph- 

 ical isolation is a 'factor or condition in 

 the formation of every species, race or tribe 

 of animal or plant * * * on the face of 

 the earth'; especially if the isolation here 

 meant implies a physical barrier, 'geo- 

 graphical or climatic,' to the continuous 

 range of closely allied forms, as the general 



context seems clearly to involve. That iso- 

 lation is an important factor no intelligent 

 biologist will be disposed 1,0 deny, that it 

 is not the only important factor, or, as 

 regards incipient species, the chief one 

 involved there is ample evidence, well- 

 known to a large number of present-day 

 investigators. 



President Jordan, however, appears to 

 have mainly in mind species, or fully segre- 

 gated forms, rather than incipient species, 

 or intergrading geographic phases; as 

 when he states, as 'practically a universal 

 rule ' : " A barrier which prevents the inter- 

 mingling of members of a species will with 

 time alter the relative characters of. the 

 groups of individuals thus separated. 

 These groups of individuals are incipient 

 species and each may become in time an 

 entirely distinct species if the barrier is 

 really insurmountable. ' ' 



In illustration of his theme, the author 

 cites examples of mammals, birds, fishes 

 and moUusks, which serve very well to illus- 

 trate the points at issue; except that in- 

 cipient species, as well as fully segregated 

 forms, are claimed to owe their existence to 

 barriers, either geographic or climatic, 

 which is not generally the case, taking the 

 term 'barrier' in its commonly accepted 

 sense. 



Doubtless President Jordan well knows 

 that among birds and mammals, and espe- 

 cially among the former, many wholly dis- 

 tinct congeneric species, or forms that are 

 known not to intergrade, are often much 

 more nearly alike, structurally and in color 

 and size, than are the extremes in many 

 groups of forms that are known to inter- 

 grade completely through geographically 

 intervening forms, as notably in the song 

 sparrow {Melospiza cinerea) group ; and 

 that many other conspecific groups of wide 

 distribution show nearly parallel varia- 

 tions, as the horned larks, and, to a lesser 



