136 EVOLUTION. OF LIFE. 
fish and Lepidosiren from the Ganoids, the Fish and 
Ascidians from some Sac-worm, the Echinodermata and 
Articulata from the Articulated Worms; finally, that the 
animal and vegetal kingdoms are the diverging stems of 
an intermediate kingdom, arising through spontaneous 
generation, or whose origin is unknown. This theory of 
the gradual descent of the higher animals from the lower 
explains perfectly why the phases exhibited in the develop- 
ment of man should be more or less permanently repre- 
sented by lower animals, or, as John Hunter expressed it, 
"If we were to take a series of animals from the most 
imperfect to the perfect, we should probably find an imper- 
fect animal corresponding with some stage of the most 
perfect." This view of nature throws light on the presence 
of rudimentary organs, such as the wings of birds and 
insects who never fly, the eyes of fish who, living in dark 
caves, never see, and the teeth of young birds and of 
certain whales who, when adult, do not have a tooth in 
their head. In the lung-breathing Vertebrata a right and 
left lung are usually present; the organization of the snakes 
and snake-like lizards exhibits the peculiarity of only one 
lung being developed, the other being rudimentary. Of 
the egg-sacs, or ovaries, of most birds, only the left is 
developed, the right being without function. Assuming 
the theory of the transmutation of species to be true, these 
rudimentary organs have a meaning, as indicating the 
ancestry of the animals exhibiting them. Important to the 
¿volutionist are, therefore, such structures as the plica 
semilunaris of the human eye, the representative of the 
third eyelid of lower animals, the external muscles of the 
human ear, the coccygeal bones composing the short tail 
of man, the vermiform appendix, etc. The monstrosities 
of the animal and vegetal kingdom are explainable from 
this point of view, the monstrosity usually consisting in 
the excessive development or deficiency of one or more 
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