230 BULLETIN OF THE 



fessor Baird's Birds of North America,* in the text of which work fre- 

 quent reference is made to the differences in size between northern and 

 southern specimens of the same species. The same author also subse- 

 quently called attention to the subject, and explicitly announced a general 

 law of geographical variation in size ; namely, a gradual decrease in size 

 in individuals of the same species with the decrease in the latitude and 

 altitude of their birth-places.t 



In some species, and throughout some entire families, climatic varia- 

 tion is more marked than in others ; generally, however, it is very 

 appreciable, and amounts, in respect to size, not unfrequently to from 

 twelve to twenty per cent J of the average dimensions of the species. 



Climatic Variation in the Bill. — The climatic variation in the size of 

 the bill is, in general, inverse to that of the general size of the individual. 

 In some species, as in the Sittce and the typical members of the Picidce, 

 I have as yet been unable to trace an independent variation in the size 

 of the bill to that of the body ; but in many species there is not only a 

 marked relative increase in the size of the bill to the southward, but, in 

 some, an absolute increase, especially in its length. 



* Pacific Railroad Explorations and Surveys, Vol. IX, Birds. By Professor S. F. 

 Baird, with the co-operation of Mr. John Cassin and Mr. George N. Lawrence. 1858. Sub- 

 sequently republished under the title of " The Birds of North America," with an Atlas 

 of one. hundred plates. 



t Proc. Phila. Acad. Nat. Sci., Vol. XI, p. 300, November, 1859. Also in Am. Journ. 

 Sci. and Arts, 2d Ser., Vol. XLT, p. 190, March, 1866. 



\ Variation in size with differences in habitat is by no means confined to birds. In 

 mammals it .is well known to be as great, if not greater, than among birds. In some 

 wide-ranging species of mammals there appears to be a double decadence in size, — a 

 diminution to the northward, in those non-migratory species whose habitats extend into 

 the arctic regions, as well as a diminution to the southwards of the point where in gen-, 

 eral the maximum of size is attained, — as I have elsewhere had occasion to remark. 

 (Bull. Mus. Comp. Zotil., Vol. I, p. 199.) But in these exceptional cases of a decline 

 in size to the northward, the cause of such a decline must result from climatic 

 conditions the i-everse of those producing the decline at the southward, — from the 

 excessive rigor of the arctic climate instead of from the enervating influence of 

 warm temperate and sub-tropical latitudes. 



In the case of reptiles, the larger representatives of a given species are generally found 

 at the North, as has also been observed to be the case with the edible marine and fluviatile 

 fishes. (I am credibly informed that this is markedly the case with the codfish and the 

 halibut.) In some groups of Crustacea and mollusca-the same fact has been repeatedly 

 observed;' but in insects, as in plants, the increase in size is generally to the southward, 

 as is especially noticeable in the diurnal Lepidoptera. In plants, however, the increase 

 is a purely vegetative one, the northern representatives of a given species being gener- 

 ally far the most prolific, in proportion to the size of the plant, utar the northern 

 limit of their respective habitats. 



