Physical phenomena we all admit follow one another in unbroken succession, each derived from a preceding, and giving origin to a succeeding. We call this the law of causation, and say that it is axiomatic. We might call it a law of derivation. So also organic forms follow one another in continuous chain, each derived from a preceding and giving origin to a succeeding. We call this a law of derivation. We might call it a law of causation, and say that it too is axiomatic. The origins of new phenomena are often obscure, even inexplicable, but we never think to doubt that they have a natural cause; for so to doubt is to doubt the validity of reason, and the rational constitution of Nature. So also the origins of new organic forms may be obscure or even inexplicable, but we ought not on that account to doubt that they had a natural cause, and came by a natural process; for so to doubt is also to doubt the validity of reason, and the rational constitution of organic Nature. The law of evolution is naught else than the scientific or, indeed, the rational mode of thinking about the origin of things in every department of Nature. In a word, it is naught else than the law of necessary causation applied to forms instead of phenomena. Evolution, therefore, is no longer a school of thought. The words evolutionism and evolutionist ought not any longer to be used, any more than gravitationism and gravitationist; for the law of evolution is as certain as the law of gravitation. Nay, it is far more certain. The nexus between successive events in time (causation) is far more certain than the nexus between coexistent objects in space (gravitation). The former is a necessary truth, the latter is usually classed as a contingent truth. I have used and may continue to use the term evolutionist, but if so it is only in deference to the views of many intelligent persons, who do not yet see the certainty of the law.
CHAPTER II.
SPECIAL PROOFS OF EVOLUTION. Introductory.
It will be seen from the preceding chapter that we regard the law of evolution in its wider sense, viz., the derivative origin of all forms, organic or other, as axiomatic, and therefore requiring no further proof. Among scientific men there is no longer any discussion of the truth of this law, but only of the theories of the causes of the law. We believe that to the scientific mind there is no other rational mode of looking at the subject of origin of organic forms. To such a mind, therefore, all that follows is but the deductive application of that law in the explanation of the phenomena of organic Nature. But it takes time for the popular mind to readjust itself to new and revolutionary truth. Many minds, even among the most intelligent, have not yet accepted this as the only rational mode of thought. Many men require further special proofs of the derivative origin of organic forms. Even to those who accept evolution, these proofs will be interesting as illustrations of such origin. We will attempt to bring out these proofs under several heads, the most important of which are: 1. Proofs from morphology, or the general laws of animal structure; 2. Proofs from embryology; 3. Proofs from geographical distribution of organic forms; and, 4. Proofs from artificial breeding. The subject is so vast that all we can do is to touch lightly only the most salient points under each of these heads; for, as we have already said, the evidence is really nothing less than the whole science of biology. Preparatory to this, however, it is necessary to bring out a little more fully than before ([page 29]), though still only in outline, the two antagonistic views, which may be called the old and the new, or the natural and the supernatural, of the origin of new organic forms, especially species.
Origin of New Organic Forms; the Old View briefly stated.—According to the old-school naturalists, species are the ultimate elements of taxonomy: genera, families, orders, etc., may gradually change their character from age to age, by the introduction of new species; but species were supposed to be substantially permanent. It was necessary to have some unit for convenience of description and classification, and this was found to be the best because most stable. As in nearly all cases of beliefs, this doctrine was held at first somewhat loosely, as a provisional and convenient view—as a good working hypothesis—but gradually, under pressure of controversy, became more strictly formulated, and, as it were, hardened into a scientific dogma, especially in the hands of Agassiz. According to this view, the first pair or pairs of each specific kind originated we know not how, but certainly at once in its present form in full perfection, and, therefore, presumably by direct creative act of Deity; and then afterward by the law of generation continued to produce others of the same pattern indefinitely. Moreover, the first one or more pairs of each kind multiplied and spread abroad in every direction, each from its own center of origin, as far as physical conditions and struggle for life with other species would allow. This idea explains tolerably well the geographical distribution of species as we now find it. For example, species on different continents are widely different, because those on each have originated independently where we now find them, and spread in all directions as far as physical conditions would allow, but could not reach other continents because of the ocean-barrier. That this is the only reason they are not there, is shown by the fact that, if they are carried there, they usually do perfectly well. Even on the same continent, for the same reason, species may be very different if separated by impassable barriers such as high mountain-chains or by climate. But wherever one group of species, originating in one place, comes in contact on the margin of their range with another group of species originating in another place, we see no evidence of transmutation of one form into another, but only substitution of one fully-formed species for another equally fully formed. Therefore, we must conclude that physical conditions may limit the range of a species, but can not transmute it into another. Thus, to say the least, many of the facts of geographical distribution are well explained by this idea of creative origin in specific centers and subsequent permanence of specific form. We say many of the facts; we will show hereafter that not all can be thus explained.
But the main question is not of geographical but of geological distribution; not distribution in space, but succession in time. Species do not continue forever. On the contrary, they have changed many times in the course of geological history. As conditions become unfavorable, species die out or become extinct, and others take their place and carry forward the life and development of the organic kingdom. Now, how do they change? According to this school of thought, here also, as in geographical distribution, they are not transmuted but replaced; here also physical conditions may destroy a species, but can not transform it into another. As species die out, others are created at once, out of hand and fully formed in their place; but in accordance with a preordained plan consistently carried out and working ever toward higher and higher conditions. Thus, life is continued on the earth by the alternation of supernatural and natural processes; by the alternate use of direct and indirect action of Deity: direct in the introduction of first pairs, indirect through the natural process of reproduction in the continuance and multiplication of the species. Each species is made according to a pattern in the Divine mind, on a sort of intellectual die, and then continues to reproduce a succession of individuals of the same pattern as if struck from the same die until the die is broken or worn out. Another die is made, of another pattern, and individuals are struck from this; and so on, throughout the whole geological history of the organic kingdom. Only, we must add that the successive dies are made to follow one another according to a plan which is expressed by the three laws already given on [page 11]. Thus, the origin of individuals is natural, the origin of species supernatural; the making of dies is supernatural, the coinage is natural.
We have stated this view in a too extreme form, in order to make it clearer. We now, therefore, proceed to qualify somewhat. Specific types were held, by writers of this school of thought, to be substantially but not absolutely unchangeable. Successive individuals of the same species were admitted to be not exactly alike. Such slight differences were called varieties. It was admitted, indeed, that species varied, but it was believed that such variations in any direction were strictly limited in amount. A species may be compared to a right cylinder standing on end. As such a cylinder may be tilted slightly in one direction or another, without overthrowing its equilibrium, the cylinder tending ever to right itself and return to its original position, so a species may be varied slightly in one direction or another without destroying its integrity, the species tending ever to return to its normal or typical form. But as the cylinder, if pushed too far from its normal position, is overthrown, so also a species, if pressed too far in the way of variation from its typical form, is destroyed, but not changed into another species. As cylinders may be more or less rigid, depending upon the breadth of their bases, so also some species are more rigidly set in their typical form, and some are more plastic to influences causing variations, but in all cases there is a limit to the amount of oscillation consistent with integrity.
The New View briefly stated.—According to Darwin, and all biologists of the present day, species are variable without limit, if only the causes of change are constant and slow enough in their operation, and the time long enough. A species must be in harmony with its environment, for this is the condition of its existence. Now, if the environment change, the species must tend to change slowly from generation to generation, so as to readjust its relations in harmony with the changing environment. If the change of environment be slow, the readjustment may be successful, and the species will change gradually into another form, so different that it will be called a different species, especially if the intermediate gradations be destroyed. If the change in the environment be too rapid, many species, especially the more rigid, will be destroyed, while the more plastic may survive by modification. Thus, at every step in the evolution of the organic kingdom, some species have died without issue, while others have saved themselves by changing into new forms in harmony with the new environment. Comparing to a growing tree, some branches overshadowed die, while others push on for light, forming new lateral buds, and dividing as they grow. By continued divergent change species gradually become genera, genera families, etc. Thus, varieties, species, genera, families, orders, classes, etc., are only different degrees of differences formed all in the same way. Varieties are only commencing species, species commencing genera, and so on. There is no making and wearing out of dies, and making of new ones; the whole process is a natural one—the whole series is genetically connected. In a perfect classification varieties, species, genera, families, orders, classes, etc., are only different degrees of blood-kinship.
So much may be regarded as certain, and out of the field of discussion among biologists of the present day. It is only in defining this process more accurately, and especially in the theory of the causes or factors of evolution, that there are still difference and discussion. The most probable view on this subject we now proceed to give.