MUSEUM OF COMPARATIVE ZOOLOGY. 197 



continues in a slight degree through life. The change of Falco candi- 

 cans from du-ky when young to nearly white when fully mature may 

 be hardly referable wholly to the blanching of age ; but the gradual 

 obsolescence of the dusky mottliugsof the snowy owl {Nyctea m'rea), a3 

 it advances in age, seems strictly parallel to the blanching of the gray 

 colt to a white horse. Hence a second law of variation in color in old 

 age, namely, that of smile decline. 



Individual Variation in General Size and in the Relative 

 Size of Different Parts. 



Individual Variation in General Size and Form. — Measurements 

 of scores of specimens of birds of the same species and sex, collected at 

 the same locality and season, show the existence of a large range of 

 individual variation, both in size and in general proportions; the varia- 

 tion extending to every external part of the body, and implying a 

 corresponding variation in the internal anatomy. In birds size has 

 usually been regarded, from its comparative constancy in the same 

 species, as an important specific character. But from the fact that 

 specimens of closely allied species often differ but little from each other 

 in this respect, it has been justly looked upon as being in some cases 

 more or less unreliable ; but from the great importance commonly 

 attached to it, it is evident that such instances are usually regarded as 

 exceptional. Individual variation in this respect having been formerly 

 regarded as too slight to have any significance, the size of a single speci- 

 men has usually been given as that of the species to which it belonged ; 

 hence subsequent variations from it discovered in other specimens of the 

 same species has sometimes led to the recognition of the latter as 

 specifically distinct. E-pecially has this .been the case when a differ- 

 ence in size has been associated with a wide difference of locality. The 

 facts in the case, however, show that a variation of fifteen to twenty per 

 cent in general size, and an equal degree of variation in the relative 

 size of different parts, may be ordinarily expected among specimens of 

 the same species and sex, taken at the same locality, while in some 

 cases the variation is even greater than this. Table A (p. 198) shows 

 to some extent the general variation in size, but it does not always 

 give, nor even generally, the extreme differences in the size of similar 

 parts, as the wing, tail, etc., since those averaging the largest or 



