322 BIOLOGY FOR BEGINNERS 



possess organs which are of no use to them, but are developed in 

 other groups for important functions. 



For example, in the foot of the horse there are unused bones 

 which in other animals support separate toes. The ostrich has 

 small wings like those of other birds, but it cannot use them for 

 flight. The boa constrictor has remnants of a hip girdle though 

 it has never developed legs to use it. 



In man there are about seventy such structures, well developed 

 in other animals but reduced in size and function in his body, 

 like remains of the scaffolding of construction left in a completed 

 building and showing thereby the process of its development. 

 Among these may be mentioned the appendix which in the rodents 

 is the largest part of the intestine, while in man it is reduced to a 

 small and apparently useless rudiment. Similarly we have small 

 canine teeth, but do not develop them to tear food like the dog; 

 we have an inturned ear tip and muscles to move it, but we do not 

 " prick up our ears " like a horse. 



The list might be greatly extended, but the point is this, if 

 animals and plants are not developed from common ancestors, 

 why then do they have these resemblances in structure. 



2. Embryological Resemblances. In the study of the develop- 

 ment of the embryos of all animals, it is found that the higher 

 forms pass through stages resembling lower types, as they develop. 



The first stage of all plants and animals is the single fertilized 

 egg cell. In all cases this develops by almost identical steps, into 

 (a) a solid mass of cells, (b) a hollow sphere of cells, (c) an infolded 

 tubular form, and then up through more and more specialized 

 structures to the adult, whatever it may be. The early forms of 

 all vertebrate embryos are so similar that dog, cat, rabbit, or man 

 cannot easily be distinguished until well started toward adult form. 



By watching embryonic development of the vertebrates we can 

 observe modifications of various structures, such as the gill arches, 

 which are present in all the early stages. These gradually develop 

 true gills in the fish, but become modified and reduced in the 

 higher forms, their rudiments appearing in man as parts of the 

 inner ear, lower jaw, and throat cartilages. Certainly, if animals 



