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life, while other such elements remained inorganic ; there 
must have been a point at which some vegetables became 
animals, while other vegetables remained vegetable ; or 
perhaps there were some common forms, neither animal 
nor vegetable, which at certain points became distinctively 
vegetable on the one hand, or distinctively animal on the 
other. There must have been a point at which some 
invertebrates began to be vertebrates, while other inverte 
brates remained invertebrate. There must have been a 
point at which, following one line of descent, some fish 
became reptiles, some reptiles amphibians, some amphibians 
mammals; and when, following another line of descent, some 
reptiles became birds. And if all this took place under 
the reign of law, there must have been mechanical, 
chemical, vital laws by which Organic Evolution has been 
brought about. But, at present, they are too dimly dis 
cerned to enable us to found any strong argument upon 
them in favour of the theory of transformation, as opposed 
to the theory of Natural Selection ; and, with one notable 
exception, I believe that no attempt has been made to 
prove that Natural Selection has been the determining 
cause of those new departures which produce such different 
results. 
But if the discussion of these topics has not proceeded 
far enough for the purposes of our present argument, the 
chasm which once appeared to divide the inorganic from 
the living, the simple forms from the more complex, the 
invertebrate from the vertebrate, the fish from the reptile, 
the reptile from the bird, or the reptile from the am 
phibian, and the amphibian from the mammalian, has 
been narrowed very considerably, and the whole process 
of Organic Evolution, represented by a tree-like classi 
fication, becomes no more difficult to conceive than the 
transmutation of one species into another.