WHAT IS EVOLUTION? 727 



but the whole tree grows ever higher in its highest parts, grander in 

 its proportions, and more complexly diversified in its structure. 



It may be well to pause here a moment to show how this mistaken 

 identification of evolution with progress alone, without modification 

 by the more fundamental laws of differentiation, has given rise to 

 misconceptions in the popular and even in the scientific mind. The 

 biologist is continually met with the question, " Do you mean to say 

 that any one of the invertebrates, such, for instance, as a spider, may 

 eventually, in the course of successive generations, become a verte- 

 brate, or that a dog or a monkey is on the highway to become a man ? " 

 By no means. There is but one straight and narrow way to the high- 

 est in evolution as in ail else, and few there be that have found it in 

 fact, probably two or three only at every step. The animals men- 

 tioned above have diverged from that way. In their ancestral his- 

 tory, they have missed the golden opportunity, if they ever had it. 

 It is easy to go on in the way they have chosen, but impossible to get 

 back on the ascending trunk-line. To compare again with the growing 

 tree, only one straight trunk-line leads upward to the terminal bud. 

 A branch once separated must grow its own way, if it grow at all. 



Of the same nature is the mistake of some extreme evolutionists, 

 such as Dr. Bastian and Professor Haeckel, and of nearly all anti-evo- 

 lutionists, viz., that of imagining that the truth of evolution and that 

 of spontaneous generation must stand or fall together. On the con- 

 trary, if life did once arise spontaneously from any lower forces, physi- 

 cal or chemical, by natural process, the conditions necessary for so ex- 

 traordinary a change could hardly be expected to occur but once in the 

 history of the earth. They are, therefore, now, not only unproduci- 

 ble, but unimaginable. Such golden opportunities do not recur. Evo- 

 lution goes only onward. Therefore, the impossibility of the deriva- 

 tion 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 verte- 

 brates. 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 reformed from any other class now living. It would be necessary 

 to go back to the time and condition of the separation of this class 

 from the reptilian stem. Therefore, the falseness of the doctrine of 

 abiogenesis,* so far from being any argument against evolution, is ex- 

 actly what a true conception of evolution and knowledge of its laws 

 would lead us to expect. 



c. The movement of evolution has ever been onward and upward, 

 it is true, but not at a uniform rate in the whole, and especially in the 

 parts. On the contrary, it has plainly moved in successive cycles. 



* Genesis without previous life spontaneous generation. 



