io8 GENERAL ORNITHOLOGY part ii 



they are by modified descent from lower organisms. Such transitory 

 stages of any embryo, therefore, give us glimpses of those evolu- 

 tionary processes which have affected the group to which it belongs. 

 Any bird, for example, when a germ, is at first on the plane of 

 organisation of the very lowest known creatures, — one of the 

 Protozoa, or single-celled animals. As its germ develops, and its 

 structure becomes more complicated by the formation of parts and 

 organs successively differentiated and specialised, it rises higher 

 and higher in the scale of being. At a certain stage very early 

 reached (for the steps by which it becomes like any invertebrate are 

 very speedily passed over) it resembles a fish in possessing gill-like 

 slits, several aortic arches, no true kidneys, no amnion, etc. Further 

 advanced, losing its gills, gaining kidneys and amnion, etc., it rises 

 to the dignity of a reptile, and at this stage it is more like a 

 reptile than like a bird ; having, for example, a number of separate 

 bones of the wrist and ankle, no feathers, etc. The assumption of 

 its own appropriate characters, i.e.. those by which it passes from a 

 reptilian creature to become a bird, is always the last stage reached. 

 We can thus actually see and note, inside any egg-shell, exactly 

 those progressive steps of development of the individual bird which 

 we believe to have been taken on a grand scale in nature for the 

 evolution of the class Av^s from lower forms of life ; and the lesson 

 learned is fraught with significance. It is nothing less than the 

 demonstration in ontogeny (genesis of the individual) of that 

 phylogeny (genesis of the race) by which groups of creatures come 

 to be. The interior of any adult bird, again, furnishes us with all 

 kinds of ordinary anatomical characters, derived from the way we 

 perceive the different organs and systems of organs to be fashioned 

 in themselves, and arranged with reference to one another. The 

 finishing of the outward parts of a bird gives us the ordinary 

 external characters, in the way in which the skin and its appendages 

 are modified to form the covering of the bill and feet, and to fashion 

 all kinds of feathers. Birds being of opposite sexes, and such 

 difference being not only indicated in the essential sexual organs, 

 but usually also in modifications in size or shape of the body or 

 quality of the plumage and other outgrowths, a set of sexual 

 characters are at our service. Birds are also sensibly modified in 

 their outward details of feathering by times of the year when the 

 plumage is changed, and this renders appreciation of seasonal 

 characters possible. All such circumstances, and others that could 

 be mentioned, such as effects of climate, of domestication, etc., in so 

 far as they in any way affect the structure of birds, conspire to 

 produce zoological characters, as these are above defined. Such 

 characters, according as they result from more or less profound 

 impressions made upon the organism, are of more or less " value " 



