10 GENERAL DEFINITIONS. 



of such terms, and however widely the adherents of 

 the modern doctrine of Evolution are at variance with 

 those who accept Agassiz's notion of the separate cre- 

 ation and fixity of species. We can only retain the 

 above definition of species with the understanding that 

 the "relation of individuals to each other" is genetic, 

 all species bearing to each other the relation of parent 

 and offspring ; and that their relation to " conditions 

 of environment " is largely one of cause and effect. 



The study of any particular group of animals con- 

 stitutes one of the special departments of zoological 

 science. Thus, Ornithology is the science of birds ; 

 and with it is generally associated Oology, or the study 

 of their eggs. Birds form a class of animals easily 

 recognized, among other characteristics of more or less 

 exclusive pertinence, by the possession of feathers 

 those peculiar out-growths from the skin which are 

 found in no other class than that of Aves. Birds oc- 

 cupy the next to the highest place in the scale of ani- 

 mal life, being only surpassed in relative rank by the 

 Mammalia, to which man himself belongs. Their 

 closest relationships, however, are with the reptiles ; 

 both birds and reptiles of the present geologic epoch 

 being believed to have descended from a common an- 

 cestry. Birds of previous periods in the world's his- 

 tory had teeth, and presented other strong reptilian 

 characters, which have gradually been lost as the two 

 branches of one common stock diverged from each 

 other in the process of evolution. Progressive speciali- 

 zation of structure and coincident differentiation of 

 function have resulted in the extremely modified creat- 

 ures we now know as birds, and produced a very 

 clearly defined and completely circumscribed class of 



