MUSEUM OF COMPARATIVE ZOOLOGY. 229 



are doubtless more or less obscurely traceable. Among these are cer- 

 tain circumstances attending the time of hatching, as well as, of course, 

 the vigor of the parent. Not unfrequently the first attempts of birds to 

 rear their brood, are unsuccessful, from their eggs or young bein<* de- 

 stroyed by their enemies. Persisting, however, in their efforts, it is late 

 in the season before their brood is fledged, several sets of eggs or young 

 having been previously destroyed. The birds of such broods are found 

 to be smaller and paler colored than those hatched earlier in the 

 season. In cases where several broods are reared each year, as a 

 general rule the birds of the earlier brood seem in all respects the most 

 perfect and vigorous. Various other causes operating during their 

 infancy doubtless more or less affect their general size, their propor- 

 tions, and colors when mature. Food has doubtless much to do with 

 variation in color, though but few facts bearing upon this point have 

 been yet recorded. Professor Agassiz informs me, however, that many 

 years since, in Switzerland, he raised many Pyrrhula vulgaris, and 

 found that by feeding them on the seeds of hemp the red on the breast 

 changed to black. The well-known fact that certain brightly colored 

 birds, as the purple finch (Carpodacus purpureus) and the crossbills 

 (Curvirostra), change, when kept in cages, from bright red to dull 

 olive with their first moult, and never again, or at least so long as kept 

 in confinement, regain their original color, shows how susceptible the 

 color of birds is to the influences of food and artificial conditions of life. 



Climatic Variation. 



Climatic variation involves as completely all parts of the animal as 

 does individual variation. It is more marked, however, in some features 

 than in others. The three most prominent phases of climatic variation 

 in birds are the following : variation in general size, variation in the 

 size ;md form of the bill, variation in color. 



Climatic Variation in Size. — Variation in the size of individuals of 

 the same species with differences in the latitude and altitude of their 

 respective places of birth is a fact already so well known as to be quite 

 generally recognized ; hence any demonstration of such a variation is 

 in the present connection unnecessary. A few tables of comparative 

 measurements of New England and Florida specimens given in Part IV 

 serve to illustrate its general character and extent. Similiar illustrations 

 are abundantly afforded by the tables of measurements published in Pro- 



