138 EVOLUTION OF LIFE. 
That which, under the ordinary laws of vegetation, would 
have developed as a leafy branch, here develops as a flower ; 
its several organs appearing under forms some of them 
slightly and others extremely different in aspect and in 
office from the foliage. But they all have a common nature 
and a common origin, or, in other words, are homologous 
parts. When, therefore, the floral organs are called modified 
or metamorphosed leaves,it is not to be supposed that a 
petal has ever actually been a green leaf, and has subse- 
quently assumed a more delicate texture and hue, or that 
stamens and pistils have previously existed in the state 
of foliage, but only that what is fundamentally one and 
the same organ develops, in the progressive evolution of 
the plant, under each or any of these various forms." The 
visceral arches of the Vertebrata are among the many illus- 
trations of this idea offered by the animal kingdom. The 
visceral arches (Figs. 178 to 181, c) consist of thickenings 
or papille situated behind the primitive eye, and below 
the primitive ear. They are present in the early stages of 
all Vertebrata, and are much modified in the course of 
development. .The branchial arches supporting the gills 
in Fishes represent best their primitive condition, while 
in remaining Vertebrata they are used partly in the 
formation of the lower jaws, partly in the formation of 
the organs of hearing. 
سے 
The subject of Embryology is as intimately related to 
Geology as to that of Anatomy, for the changes through 
which plants and animals pass in the course of develop- 
ment are essentially the same changes through which life 
in general has passed from its first appearance to the pres- 
ent time, for not only are the transitory stages of the 
higher animals permanently represented by the lower, but 
they are also permanently represented by the fossils. In 
other words, the development of the most complex plant, 
and of the most highly-organized animal, is an epitomized 
