Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/186

136 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 development of man should be more or less permanently represented 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 imperfect 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 Evolutionist 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