progress from the germ to the adult stage present themselves to view as simply meaningless facts and useless freaks and vagaries of Nature. Accepting the idea—favored, one may add, by every circumstance of life-science—much that was before wholly inexplicable becomes plain and readily understood. And the view that a living being's development is really a quick and often abbreviated summary of its evolution and descent both receives support from and gives countenance to the general conclusion that life's forces tend as a rule toward progress, but likewise exhibit retrogression and degeneration. If a living being is found to begin its history, as all animals and plants commence their existence, as a speck of living jelly, comparable to the animalcule of the pool, it is a fair and logical inference that the organisms in question have descended from lowly beings, whose simplicity of structure is repeated in the primitive nature of the germ. If, to quote another illustration, the placid frog of to-day, after passing through its merely protoplasmic stage, appears before us in the likeness of a gill-breathing fish (Fig. 1), the assumption is plain and warrantable that the frog
Fig. 1.—Development of Frog.
race has descended from some primitive fish stock, whose likeness is reproduced with greater or less exactness in the tadpoles of the ditches. Or if, to cite yet another example, man and his neighbor quadrupeds (Fig. 2), birds, and reptiles, which never breathe by gills at any period of their existence, are found in an early stage of development to possess "gill-arches" (g), such as we naturally expect to see, and such as we find in the fishes themselves, the deduction that these higher animals are descended from gill-bearing or aquatic ancestors admits of
