EVOLUTION OF THE COLORS OP BIRDS. 77 



figuratively, or in other words, in the great multiplica- 

 tion of such species as already existed. In course of 

 time the conditions of life became less favorable to 

 them, more powerful antagonists arose, and they began 

 to dwindle. Retardation began, and to-day there are 

 left a few poor fragments of that mighty host that lived 

 in the age of reptiles. 



Prof. Alpheus Hyatt's law of concentration is very 

 closely related to the preceding, being in fact a form of 

 acceleration. Prof. Hyatt thus alludes to it:* "The 

 law of concentration in development seems to express 

 an invariable mode of action of heredity, in the earlier 

 reproduction of hereditary characteristics of all kinds 

 and under all conditions. In progressive series it acts 

 upon healthy characteristics and appears to be an 

 adaptation to favorable surroundings, and in retrogress- 

 ive series upon pathological characteristics, and is prob- 

 ably an adaptation to unfavorable surroundings usually 

 leading to the extinction of the series or type." 



This ends the discussion of the laws of development. 

 Of the laws of structure and heredity little need be said. 

 For the sake of completeness, however, and especially 

 as it will be necessary to refer to them in the second part 

 of the work, it will be advisable to include a brief state- 

 ment of them. Prof. Cope, in the Chapter on Evolution 

 and its Consequences,! states the following four laws of 

 structure: 



" 1. Homology. This means that animals are com- 

 posed of corresponding parts; that the variations of an 

 original and fixed number of elements constitute their 

 only difference. * * * 



" 2. Successioncd Relation. This expresses the fact 



* Fossil Cephalopoda in the Museum of Comparative Zoology. Proo. 

 A. A. A. S., 1883, xxxii, p. 360. 

 tl. c, pp. 6-7. 



