It is obvious that the pros and cons for either course must be supposed very nearly equal, otherwise the course which presented the fewest advantages would be attended with the probable gradual extinction of the organised beings that adopted it, but there being supposed two possible modes of action very evenly balanced as regards advantage and disadvantages, then the ultimate appearance of two corresponding forms of life is a _sequitur_ from the admission that form varies as function, and function as opinion concerning advantage. If there are three, four, five, or six such opinions tenable, we ought to have three, four, five, or six main subdivisions of life. As things are, we have two only. Can we, then, see a matter on which opinion was likely to be easily and early divided into two, and only two, main divisions—no third course being conceivable? If so, this should suggest itself as the probable source from which the two main forms of organic life have been derived. I submit that we can see such a matter in the question whether it pays better to sit still and make the best of what comes in one’s way, or to go about in search of what one can find. Of course we, as animals, naturally hold that it is better to go about in search of what we can find than to sit still and make the best of what comes; but there is still so much to be said on the other side, that many classes of animals have settled down into sessile habits, while a perhaps even larger number are, like spiders, habitual liers in wait rather than travellers in search of food. I would ask my reader, therefore, to see the opinion that it is better to go in search of prey as formulated, and finding its organic expression, in animals; and the other—that it is better to be ever on the look-out to make the best of what chance brings up to them—in plants. Some few intermediate forms still record to us the long struggle during which the schism was not yet complete, and the halting between two opinions which it might be expected that some organisms should exhibit. “Neither class,” I said in “Alps and Sanctuaries,” “has been quite consistent. Who ever is or can be? Every extreme—every opinion carried to its logical end—will prove to be an absurdity. Plants throw out roots and boughs and leaves; this is a kind of locomotion; and, as Dr. Erasmus Darwin long since pointed out, they do sometimes approach nearly to what may be called travelling; a man of consistent character will never look at a bough, a root, or a tendril without regarding it as a melancholy and unprincipled compromise” (New edition, p. 153).