The Two Principal Theories
The first opinion is, that each species is, in its essential nature, and has been throughout its whole history, entirely distinct from every other one, and that it was called into being in its own appointed time, either by a special act of creation exerted for this end, or else by the operation of some general laws to us wholly unknown, by which, when certain conditions are combined, a new species is derived in some mysterious way from one or more other species existing before it, just as individuals of any given species are known to proceed from other individuals of the same. This opinion has been hitherto a prevailing one among naturalists and philosophers, and a great desire has been felt to discover the general conditions and laws, if such there are within the reach of human observation, under which new species arise.
The second opinion is, that life, in all its manifestations, throughout the whole vegetable and animal world, is one, and that all organizations that now exist, or have ever existed, have been produced, by a succession of exceedingly gradual and long-continued changes, from one, or at most a very few, primordial forms.
These changes, it is supposed, result from a constitution of vegetable and animal life such that very slight modifications of structure occur in all cases in the descent from parent to offspring; that these modifications, which are insignificant, and sometimes scarcely perceptible in the first generation, become very great by being accumulated in a long series of years, and that changes thus resulting, branching off in different directions, as it were, according as the conditions and influences to which different races are exposed, vary, in different places and time, and acting through immensely long periods of time, have given rise to all the countless forms of animal and vegetable life with which the world now teems.
Inquiries Into This Subject Right and Proper
This is not the place to discuss, nor even to explain these opinions. They are only briefly alluded to here, on account of the bearing of this general question on the origin of life in America. Some persons feel a degree of hesitation in following the guidance of naturalists in their inquiries in respect to the laws of life, as if the object of those engages in these studies was to discover some way of accounting for the works of creation without acknowledging the hand of a creator. But this is not so. Scientific inquiries into the causes of what we see are not attempts to dispense with a divine agency in nature, but to discover the manner in which this agency is exercised, and the laws by which it regulates it action. When Franklin, and the other philosophers of his time, made known to the world that they had discovered the cause which produced thunder and lightning, many people thought it was impious for them to pretend to have done so. For the philosophers to attribute a phenomenon which had always been regarded as produced directly by the power of God to petty secondary causes, which they had themselves discovered, was, in the opinion of these persons, atheistical and profane.
But it is now universally admitted that such a discovery does not limit or control the power of God at all. It only enables us to see somewhat further into his ways. No one detracts from the honor due to an engineer for any grand result that he produces, by explaining the mystery of the secret mechanism that he has contrived by which to produce it.
It is so with all the works of nature. We may push our inquiries in every direction with the utmost diligence and vigor, and carry them to any extent, without the least fear of ever making any discoveries which will tend in the smallest degree to supersede the agency of a supreme and all-pervading power, either in the original constitution of nature, or in the constant control of all that takes place under the operation of its laws.
The Testimony of Scriptures
There is another source of apprehension, of a religious nature, by which the mind is sometimes restricted and hampered in studying the laws of nature and the past history of the globe, and that is the fear that something will be found which may conflict, or at least appear to conflict, with the testimony of Scripture, and thus shake the foundation of our Christian faith. But we must consider that the book of revelation is intended to instruct us solely in moral and spiritual truths, while the book of nature has been opened before us to teach us science and philosophy. They are both equally from God. In one as much as in the other, it is his voice that we hear, and his instructions that we receive; and we must not allow our ears to be closed, or our reason to be trammeled, in respect to what he teaches us directly in one, by too literal interpretations of what is said incidentally and indirectly in the other. Since the great mistake which was made in the time of Galileo, when it was attempted to shut out from mankind the evidences presented by mathematics and astronomy, in respect to the laws of the solar system, by inferences ignorantly drawn from incidental allusions in the Scriptures to the motions of the heavenly bodies, all wise and good men have come to the conclusion that we must look to the word of God for instruction in moral and religious truth alone, while for science and philosophy we must go to that other volume - the great system of creation and providence - which the same infallible teacher has spread open before us. Each comes from the same hand, and each in its own sphere it, in a certain sense, equally, for us, the word of God.
Means of Transportation for Animals and Plants
A great many very curious modes by which plants and animals may be transported from one country to another, even across wide and deep seas, have recently been brought to light, which very much diminish the difficulty of supposing that America might have been stocked from the old - provided always, we grant that plants and animals are subject to extensive modifications in the course of long periods of time, by which the species is finally changed, and new forms adapted to new situations and conditions are developed.
In the first place, the sea, instead of lying motionless, except so far as it is agitated by winds, as is often supposed, is subject to a great number and variety of currents, flowing in all directions, many of them at the rate of from twenty to sixty miles a day. These currents convey fields of ice, masses of drift wood, branches of trees with nuts, fruits, or other capsules containing seeds attached to them, and the bodies of dead birds, with seeds in their crops. There are many savage nations, living in countries that produce no trees, that depend on drift wood altogether for all the material of this sort that they use in making utensils and weapons, and even sometimes for building and for fuel. Now, the trunk of a single tree might contain the seeds and eggs of a hundred different species of minute plants and animals, and though great numbers would doubtless perish, many would probably be preserved.
Experiments have recently been made to ascertain how long seeds can remain submerged in sea water without losing their power of germination, and it was found that out of many hundreds subjected to the trial quite a large number grew after being in the water from twenty to ninety days. This would give them time to be conveyed a great distance by a current of the sea flowing at the rate even of twenty-five miles a day.
A certain philosopher wishing to ascertain how far aquatic birds might convey seeds from one lake or pond of fresh water to another, in the mud adhering to their feet, took out a portion of such mud, in order to ascertain how far it might be supplied with the germs of vegetable life. The quantity which he took was about a tea-cup full. This mud he placed in a situation to allow the seeds which it contained to germinate, and as fast as little plants appeared he pulled them out and counted them. He obtained from this single tea-cup full of soil more than two hundred living plants ! Thus great numbers of transfers of plants from one region to another are doubtless made, merely by the feet of aquatic birds.