132 CALIFORNIA ACADEMY OP SCIENCES. 



which it is a preparation. The idea which I have 

 attempted to emphasize in the preceding pages is that 

 life has evolved in accordance with tolerably definite 

 and unvarying laws, and that the element of chance, .if 

 any such there be, is a very limited one. The living 

 forms of to-day are looked upon as the result of the 

 interplay of organism and environment, progress being 

 urged on by the laws of development and held in check 

 by the laws of heredity; while the organic cosmos is 

 under the ceaseless and untiring scrutiny of natural 

 selection, which, like a wise mother, is perpetually pass- 

 ing judgment on the acts of her children and abmonish- 

 ing them when they go astray. 



II. The Colors op North American Birds. 



ON MODES OF PLUMAGE CHANGES. 



I am not aware of any important addition to the sub- 

 ject of the changes of plumage in birds since William 

 Yarrell, in 1835, published a paper entitled " Observa- 

 tions on the laws which appear to influence the Assump- 

 tion and Changes of Plumage in Birds."* He there 

 distinguished four " modes by which changes in the 

 appearance of the plumage of birds are produced ": 



(1.) "By the feather itself becoming altered in 

 color." 



(2.) "By the bird's obtaining a certain number of 

 new feathers without shedding any of the old ones." 



(3.) " By an entire or partial moulting, at which old 

 feathers are thrown off and new ones produced in their 

 places." 



(4.) " By the wearing off of the lengthened lighter 

 colored tips of the barbs .of the feathers on the body, by 

 which the brighter tints of the plumage underneath are 

 exposed." 



* Trans. Zool. Soc. of London, i, pp. 13-21. 



