666 



SCIENCE. 



[N. S. Vol. XXII. No. 569. 



widely dispersed species of squirrels, sper- 

 mophiles, voles, field rats and mice, hares, 

 gophers, pouched rats and mice, deer, 

 shrews, moles and bats, afford admirable 

 illustrations of differentiation without iso- 

 lation. To cite a single notable case, the 

 Virginia deer ranges over eastern North 

 America from New England and southern 

 Canada to Central America, varying great- 

 ly but gradually, in size and other features, 

 till in the southern forms the size is only 

 about one half of that of the northern, 

 with disproportionately reduced antlers. 



Variations of this nature, however strik- 

 ing, can not be due to isolation, for in the 

 cases here in view there is no isolation, but 

 continuous distribution, and consequently 

 complete intergradation. The amount of 

 differentiation, between the more extreme 

 phases in various directions, is great 

 enough, were these several forms isolated, 

 or separated by geographic intervals, to 

 warrant their recognition as well-marked 

 species. All that is requisite to constitute 

 them species is the extinction, through 

 some physical cataclysm or other cause, of 

 the connecting links, over portions of the 

 intermediate areas by which even the ex- 

 tremest phases are at present connected. 

 Doubtless in the past many species, and 

 perhaps genera, have had their origin in 

 the dismemberment of such groups of in- 

 cipient species, through violent physical 

 changes in the distribution area of widely 

 dispersed specific types. 



At present these connecting links be- 

 tween the leading phases of highly diversi- 

 fied conspecific groups are a nuisance and 

 a stumbling block to the systematist in 

 labeling collections, since between each 

 properly namable form there is an area, 

 more or less extended, where none of the 

 forms are typically represented, but which 

 is occupied by troublesome intergrades, 

 approaching one form, usually, more than 



another, and in various degrees. These 

 intergrades, furthermore, offer . a tempta- 

 tion to aspiring young naturalists to give 

 them a name, thus 'bridging the difficulty 

 by doubling it.' If these areas of inter- 

 gradation could be transformed into areas 

 of isolation many problems in nomencla- 

 ture and much trouble in identifying speci- 

 mens would be eliminated. 



The facts already set forth explain why 

 two or more subspecies are never found in 

 the same breeding area— a proposition dis- 

 cussed at some length in the paper here 

 under consideration. They also show why 

 insular forms, if separated somewhat re- 

 motely from the parent stock, assume the 

 character of species rather than subspecies, 

 and owe their existence to isolation. But 

 insular forms exhibit various degrees of 

 distinctness from the parent stock, depend- 

 ing upon the completeness and duration of 

 the period of isolation. Theoretically there., 

 can be no intergradation between insular^ 

 forms, particularly in land mammals — not 

 so clearly in the case of birds, with their 

 exceptional powers of locomotion; but in 

 the case of forms inhabiting islands but 

 little remove*d from a neighboring main- 

 land, the environing conditions may be so 

 similar that not time enough has elapsed 

 to develop a well-marked type through the 

 action of isolation, in which case the nor- 

 mal range of individual variation in the 

 insular and the stock forms may be suffi- 

 cient to cause an overlapping of the char- 

 acters through occasional individual aber- 

 ration. This might be mistaken for actual 

 intergradation, while it is in fact merely an 

 overlapping through individual variation. 

 To this condition is probably due the recent 

 tendency to give slightly differentiated in- 

 sular forms the rank of subspecies rather 

 than that of full species, to which theoret- 

 ically they should be entitled. 



I have long been a believer, in common 



