Von Baer's formula of embryonic development, which he regarded as a progress from the apparently simple to the obviously complex, and as the individual's condensed and modified recapitulation of racial history, accentuated and stimulated a thought already existing in Spencer's mind, and in part expressed. It gave objective vividness to the concept of development which Spencer had already realised in regard to societary forms. In 1864 he wrote to G. H. Lewis, "If anyone says that had von Baer never written I should not be doing that which I now am, I have nothing to say to the contrary—I should reply it is highly probable."
Herbert Spencer spoke of his early recognition of von Baer's law as one of the moments in his intellectual development. He realised objectively and vividly that out of an apparently simple and homogeneous stage of development, there is developed by division of labour and other processes, a wondrous complexity of nervous, muscular, glandular, skeletal, and connective tissues or organs, as the case may be. Organic development is not like crystallisation; it is heteromorphic crystallisation, so to speak. From a group of apparently similar cells, heterogeneous tissues and organs are developed. Thus von Baer as an embryologist gave Spencer as a general evolutionist a concrete basis for the concept of development which was simmering in his mind.
Von Baer's Law.—It does not appear, however, that Spencer ever read von Baer's embryological memoirs, else he might have been less well-satisfied with summing up individual development as a progress from homogeneity to heterogeneity. Von Baer was much more cautious than some of his followers and expositors, and subsequent research has justified his caution. The once popular "Recapitulation Doctrine" that a developing organism "climbs up its own genealogical tree," that "ontogeny recapitulates phylogeny," is now seen to be true only in a very general way, and with many saving clauses. The germ is now known as a unified mosaic of ancestral contributions, as a multiplex of potentialities; it is even visibly very complex and anything but homogeneous or "simple"; and the individual recapitulation of racial history is verifiable rather in the stages of organogenesis than in the history of the embryo as a whole. Thus while all are agreed that there is a gradual emergence of the obviously complex from the apparently simple, that development means progressive differentiation and integration, and that past history is in some measure resumed in present development, it must also be allowed that germ-cells are microcosms of complexity, that development is the realisation of a composite inheritance, the cashing of ancestral cheques, and that the "minting and coining of the chick out of the egg" is not adequately summed up as "a progress from homogeneity to heterogeneity."
But although embryology does not appear to us to give unequivocal support to Spencer's formula of progress from the homogeneous to the heterogeneous, it seemed all plain sailing to him, and he proceeded to illustrate the utility of his formula by applying it to all orders of facts. In a famous passage in the essay on "Progress: its Law and Cause" (Essays, vol. i., 1883, p. 30) he wrote as follows:—
"We believe we have shown beyond question that that which the German physiologists (von Baer, Wolff, and others) have found to be the law of organic development (as of a seed into a tree and of an egg into an animal) is the law of all development. The advance from the simple to the complex, through a process of successive differentiations (i.e. the appearance of differences in the parts of a seemingly like substance), is seen alike in the earliest changes of the Universe to which we can reason our way back; and in the earlier changes which we can inductively establish; it is seen in the geologic and climatic evolution of the Earth, and of every simple organism on its surface; it is seen in the evolution of Humanity, whether contemplated in the civilised individual, or in the aggregation of races; it is seen in the evolution of Society in respect alike of its political, its religious, and its economical organisation; and it is seen in the evolution of all those endless concrete and abstract products of human activity which constitute the environment of our daily life. From the remotest past which Science can fathom up to the novelties of yesterday, that in which Progress essentially consists is the transformation of the homogeneous into the heterogeneous." This was written in 1857.
As far back as 1852 Spencer contributed to the 'Leader' an essay on the 'Development Hypothesis' which is one of the most noteworthy of the pre-Darwinian presentations of the general idea of evolution. Supposing that there are some ten millions of species, extant and extinct, he asks "which is the most rational theory about these ten millions of species? Is it most likely that there have been ten millions of special creations? or is it most likely that by continual modifications, due to change of circumstances, ten millions of varieties have been produced, as varieties are being produced still?... Even could the supporters of the Development Hypothesis merely show that the origination of species by the process of modification is conceivable, they would be in a better position than their opponents. But they can do much more than this. They can show that the process of modification has effected, and is effecting, decided changes in all organisms subject to modifying influences.... They can show that in successive generations these changes continue, until ultimately the new conditions become the natural ones. They can show that in cultivated plants, domesticated animals, and in the several races of men, such alterations have taken place. They can show that the degrees of difference so produced are often, as in dogs, greater than those on which distinctions of species are in other cases founded. They can show, too, that the changes daily taking place in ourselves—the facility that attends long practice, and the loss of aptitude that begins when practice ceases—the strengthening of passions habitually gratified, and the weakening of those habitually curbed—the development of every faculty, bodily, moral, or intellectual according to the use made of it—are all explicable on this same principle. And thus they can show that throughout all organic nature there is at work a modifying influence of the kind they assign as the cause of these specific differences; an influence which, though slow in its action, does, in time, if the circumstances demand it, produce marked changes—an influence which, to all appearance, would produce in the millions of years, and under the great varieties of condition which geological records imply, any amount of change."
While Spencer did not discern the modifying influence of Natural Selection, which it was reserved for Darwin and Wallace to disclose, his clear presentation of the general doctrine of evolution seven years before the publication of the "Origin of Species" (1859) should not be forgotten.
In other essays before 1858 and in his Principles of Psychology (1855), Spencer championed the evolutionist position, and the first programme of his "Synthetic Philosophy" was drawn up in January 1858.
Arguments for the Evolution-Doctrine.—The idea that the present is the child of the past and the parent of the future, that what we see around us is the long result of time, that there has been age-long progress from relatively simple beginnings—the evolution-formula in short—is now part of the intellectual framework of most educated men with a free mind. We no longer trouble to argue about it; like wisdom it is justified of its children. It has afforded a modal interpretation of the world's history, an interpretation that works well, which no facts are known to contradict. It has been the most effective organon of thought that the world has known; it is becoming organic in all our thinking.
We cannot indeed give an evolutionary account of the origin of life, or of consciousness, or of human reason; we cannot read the precise pedigree of many of the forms of life; we are in great doubt as to the modus operandi by which familiar results have been brought about, but all this ignorance does not diminish our confidence in the scientific value of the general evolution-idea. It may be that there are some primary facts, such as life and consciousness, which we must be content to postulate as at present irresoluble data, but it is also certain that our inquiry into the factors of evolution is still very young. So much has been done in half a century, since serious ætiology began, that it is premature to say ignorabimus where we must confess ignoramus.
It seems possible to give a provisional evolutionist account of so many of "the wonders of life," as Haeckel calls them, that there are few nowadays who will maintain that, given certain postulates, a scientific interpretation of nature is impossible. This is what the doctrine of special creation or creations implies; it means an abandonment of the scientific interpretation of nature as a hopeless task.