Of the most serious difficulties in the way of a theory of descent by evolution, the first concerns the nature of the evidence, inasmuch as all change coming under observation does not indicate progression or improvement in the organism. The importance of this may be best indicated by quoting Mr. Darwin's explanatory words as to alterations in the fifth edition of his book on species. He says, "In the earlier editions of my Origin of Species, I probably attributed too much to the action of natural selection or survival of the fittest. I have altered the fifth edition of the Origin so as to confine my remarks to adaptive changes of structure. I had not formerly sufficiently considered the existence of many structures, which appear to be, as far as we can judge, neither beneficial nor injurious, and this I believe to be one of the greatest oversights as yet detected in my work." When it is certain that deviation from the normal structure may take place which is a disadvantage to the individual, and that this may descend to offspring; when it is also shown that deviation may occur which appears to serve no end, that is, contributes to no phase of functional activity; when besides advantages gained are lost, and the race returns to its original type of structure; and when farther there are examples of degeneration, as in parasitic races,—such facts interpose special difficulties in the way of an all-embracing theory of progress by natural selection. Besides, as deviations occur of an unfavorable kind among domesticated animals under the care of man, it becomes obvious that progress may be readily lost even in most favorable circumstances.
The next outstanding difficulty is that of meeting the requirements of logical inference. This has been specially urged by Mr. Mivart as bearing upon the "incipient stages" of advance, and the difficulty certainly presses heavily at that point. Natural selection may account for much in the history of higher organisms where powers of sensibility and locomotion are great, but how can we find in natural selection an adequate explanation of progress in organisms within which these powers are at the lowest. The difficulty is to get a cause sufficient to account for the start of a movement so vast as that which is to culminate in man. Mr. Darwin feels the force of this difficulty, and replies thus, "as we have no facts to guide us, speculation on the subject is almost useless."[BN] But this perplexity which is most glaring at the beginning of the upward course, clings to the theory at every stage in the combination of struggle and improvement,—descent involving a real ascent. Whether the organism be more or less complex, it depends upon external causes for its improvement, and the dependence continues at every stage. Granting that there is everywhere struggle for existence and survival of the fittest, are these sufficient to account for results so great as are involved in unceasing advance of organism?
If it be argued that they are sufficient, a serious perplexity comes from the opposite quarter,—How does it happen that all organic existence does not advance together to a common elevation? If the theory accounts for advance, how shall we account for the want of it? The difficulties are as great for the theory in view of the large body of facts it does not attempt to include, as in the facts it strives to embrace. Agassiz put this difficulty with much force in 1857, and it has not received any satisfactory answer. He said, "It is a fact which seems to be entirely overlooked by those who assume an extensive influence of physical causes upon the very existence of organized beings, that the most diversified types of animals and plants are everywhere found under identical circumstances."[BO] If, as Mr. Darwin says, "looking to the first dawn of life," we may believe that "all organic beings presented the simplest structure"; if the struggle for existence is uniformly encountered and leads to survival of the fittest, how is it that within the same area, organism has not advanced to similar complexity? If under constraint of the evidence for the theory, we surrender the doctrine of inevitable fixedness of species, how shall we nevertheless account for the permanence of species? It is not suggested that there is at successive stages "new and simple forms continually being produced by spontaneous generation." This hypothesis of Lamark is rejected by Mr. Darwin,[BP] as it is by almost all scientific observers? How, then, can we explain the facts? We are told that certain orders have "fallen out" in the march of progress; but we need a scientific account of this which shall harmonize with a theory of action of environment, and such an account is not forthcoming. It may be said that the very success of the theory by accumulation of most striking and important evidence, is bringing it into difficulty, and suggesting its insufficiency. The more powerful and imposing the action of the law of natural selection, the more pressing becomes the need for a scientific explanation, at once distinct and harmonious, which will account for the persistence of species, when struggle for existence goes on under similar or even analogous action of environment. The presence everywhere of these lower forms alongside of the higher, adds greatly to the attractiveness of nature, and not even the grandeur of a universal advance towards the higher levels of organization would make up for the disappearance of the marvels of lower orders of animals. A monotony of grandeur may compare unfavorably with the wealth of variety and adversity; and so a law of continuity or persistence may be found adding to the greatness of a universe in which a law of progress or evolution also finds uniform application.
Upon this contrast between persistence and progress, general attention will henceforth be concentrated in judging of the place and value of a theory of descent. There is no need for hurry or impatience in this matter. The words of Mr. Darwin will find ready assent as he says, "No one ought to feel surprise at much yet remaining unexplained on the origin of species, if we make due allowance for our profound ignorance on the mutual relations of the inhabitants of the world at the present time, and still more during past ages."[BQ] In accordance with this acknowledgment, a wide range of scientific research still remains to be undertaken, and religious thought can have nothing but friendly interest in the work, as it may well be assured of drawing thence fresh contributions of great value for higher speculation concerning the government of the universe.
FOOTNOTES:
[AO] Tait's Recent Advances, p. 23.
[AP] Lyell's Elements of Geology, p. 2.
[AQ] Recent Advances, p. 24.
[AR] Doctrine of Descent, p. 20.
[AS] Ib. p. 161.