52 



ORNITHOLOGIST 



[Vol. 16-No. 4 



species were straiicleil on mountain tops, and 

 found tliere a compensation in altitude for tlie 

 higher latitude sought by their species in gen- 

 eral, thus forming examples of isolation found 

 to-day. A clear conception of the effects of iso- 

 lation of this kind is also a great help in under- 

 standing the theory of varietal forms which is 

 now generally accepted by American and other 

 ornithologists; to them a species and a variety 

 are in many cases intimately connected, mak- 

 ing it difficult for an expert and impossible 

 for others to distinguish the difference. 

 Briefly, when birds from remote localities, 

 though differing considerably, are found to 

 be connected by individuals blending grad- 

 ually in the distinctive characteristics of the 

 types, and inhabiting an intervening territory, 

 the farthest removed types are termed varie- 

 ties with often one or more "variety" being 

 acknowledged in the neutral ground. If, on 

 the contrary, the birds from the farthest re- 

 moved localities were found to be unconnected 

 by the grading of intermediate forms, then 

 each vpould be considered a valid species. 

 Again, supposing the case of four types in 

 this imaginary area, one of which graded 

 imperceptibly over the intervening country 

 until identical with the second, and where be- 

 tween the third and fourth types there was no 

 intergradation: even though the two without 

 intermediate forms varied less, one from 

 another, than did the extremes of the inter- 

 grading set, the same rule of specific and 

 varietal relation would hold good. 



Supposing the existence of a case similar to 

 the above (and many do exist in our own 

 country) and imagine the gradual change of 

 the intervening country in such a way as to ex- 

 terminate the grading forms, as for example, 

 the encroachments of an ocean, the changing 

 from a fertile country to a state of aridity, or 

 a different climate brought about by geologi- 

 cal agency, we would finally have two species 

 without intergrading forms, or in other words, 

 two species would have been created from 

 what was previously one type of several forms. 

 If we are to believe the testimony of tlie 

 rocks this has happened many times over, for 

 fo.ssils showing intermediate structure to later 

 or living forms .are not wanting. 



The growth of families, genera and species 

 in this way aided by a constant individual 

 variation, allowing those best constituted for 

 life amid the surrounding influences to live and 

 reproduce their kind, while those less ably en 

 (lowed died out, show by their remains left 

 fossil in the rocks, a clear object lesson in sup- 



port of this theory. The study of the effects of 

 the gl.acial period on the various forms of 

 animal and vegetable life, and its pronounced 

 effects on various ornithological phenomena 

 will be mentioned in a future paper on 

 migration. Harry Gordon Wliite. 



U. S. Fisli (.'onimission, Wood's Holl, Mass. 



A Series of Eggs of the Black- 

 throated Green Warbler. 



The eggs of the Black-throated Green War- 

 bler (Dendroica virenn) do not show as great a 

 variatitm as those of many other species of the 

 same genus. 



Set I. .June 17, 1887. Grand Manan, N.B. 

 Nest on spruce tree, two feet from ground. 

 Four eggs, fresh. White, with a faint creamy 

 tinge, spotted with purplish-gray and russet. 

 The markings form wreaths around the larger 

 ends. (This appears to be characteristic of 

 the eggs of this bird, and in describing the 

 .sets menti<med below it will be understood 

 that the markings are so arranged, unless 

 otherwise mentioned.): .r)Ix.4!l: .(!lx.4il; 

 .(!l X.4S); .(!0x.4S. 



Set n. June 19, 1888. Grand Manan, X. B. 

 Nest on si)ruce tree, nine fett from ground. 

 Four eggs, incubation commenced. White, 

 speckled with purplish-gray and burnt umber. 

 Although the markings form wreaths around 

 the larger ends as usual, still there are ijuite a 

 number of specks scattered ovei- the rest of 

 the surface of the eggs: .62x.4'.t; .r]8x.4!l; 

 .(5lx.4i); .111 X.411. 



.Set III. .lune 2."), 1888. Grand Manan, N. B. 

 Nest on alder tree, five feet from the giound. 

 Four eggs, incubation commenced. White, 

 spotted with purplish-gray and russet: .00x.4il; 

 .66X..52; .(i2x.4H; .(iSx.-M). 



Set IV. June HI, 1888. Grand Manan, N. B. 

 Nest on spruce tree, four and a half feet from 

 the ground. Four eggs, incubation com- 

 menced. White, s]iotted with purplish-gray, 

 and burnt umber. All are wreathed, but one 

 egg has spots all over the surface: .70x.40; 

 .70X.48; .OOx.150; .70x.4»). 



Set V. June 14, 1887. Grand Manan, N. B. 

 Nest on alder tree, five and a half feet fnmi 

 the ground. Four eggs, fresh. White, spotted 

 with burnt umber and purplish- gray : ..59 x .47; 

 .63X.49; .01 X.49; .0:3x.50. 



Set VI. June 8, 1890. Farmlngton, Maine. 

 Nest in spruce tree, twenty-five feet from the 

 ground. Four eggs, small embryos. White, 

 with a very faint bluish tinge, speckled with 



