verse are in the sense explained wholly fortuitous and might as well have all been different from what they are, it is a legitimate question to inquire whether there remains anything which is not thus fortuitous, and which must in the nature of things be what it is. And we find that there are such things. There are essentials as well as accidents, but they belong to a different category. If we examine the matter closely, we will see that all the cases considered come under the head of form—worlds, plants, animals, men. But there is another great class of cases which fall under the head of forces or principles, and these when carefully examined are found not to be variables but constants—the constants of nature. By this I do not mean that they always exist at all times and places, although this is probably true of the universal gravitant and radiant forces, of which, indeed, all the other forms of energy are doubtless special conditions. I refer in general to what is known as the principle or law of evolution, and in particular to the three latest phases of that law which are called respectively. Life, Feeling and Thought. For while the forms through which these modes of energy are manifested may vary to any required extent, I cannot conceive that the attributes themselves could under any circumstances be other than they are. For example, while the fancied inhabitants of Mars might all differ in every other particular from those of this earth, it is impossible to conceive them as not endowed with life at least, although we can suppose them devoid of feeling in the same sense that we conceive plants to be. But if we imagine them to have advanced even to the lowest animal stage we are obliged to endow them with feeling, consciousness, will. And when we speak of a remote planet being "inhabited," although we can abstract from those inhabitants every physical character that belongs to man and conceive them as dragons, or satyrs, or monsters of any form, we cannot imagine them devoid of reason and intelligence in addition to the attributes of life and sensibility.
Coming back to earth and confining ourselves to what we actually know, we thus see that three great steps in evolution have been taken since the surface of our globe became firm
