Of the same nature is the mistake of some extreme evolutionists, such as Dr. Bastian and Professor Haeckel, and of nearly all anti-evolutionists, viz., that of imagining that the truth of evolution and that of spontaneous generation must stand or fall together. On the contrary, _if_ life did _once_ arise spontaneously from any lower forces, physical or chemical, by natural process, _the conditions necessary for so extraordinary a change could hardly be expected to occur but once in the history of the earth_. They are, therefore, _now_, not only unreproducible, but unimaginable. Such golden opportunities do not recur. Evolution goes only onward. Therefore, the impossibility of the derivation of life from non-life _now_, is no more an argument against such a derivation _once_, than is the hopelessness of a worm ever becoming a vertebrate _now_, an argument against the derivative origin of vertebrates. Doubtless if life were now extinguished from the face of the earth, it could not again be rekindled by any natural process known to us; but the same is probably true of every step of evolution. If any class--for example, mammals--were now destroyed, it could not be re-formed from any other class now living. It would be necessary to go back to the time and conditions of the separation of this class from the reptilian stem. Therefore, the falseness of the doctrine of abiogenesis,[7] so far from being any argument against evolution, is exactly what a true conception of evolution and knowledge of its laws would lead us to expect.