And now I must once more point out that a grave responsibility rests on biologists in respect of the general question; since wrong answers lead, among other effects, to wrong beliefs about social affairs and to disastrous social actions. In me this conviction has unceasingly strengthened. Though The Origin of Species proved to me that the transmission of acquired characters cannot be the sole factor in organic evolution, as I had assumed in Social Statics and in The Principles of Biology, published in pre-Darwinian days, yet I have never wavered in the belief that it is a factor and an all-important factor. And I have felt more and more that since all the higher sciences are dependent on the science of life, and must have their conclusions vitiated if a fundamental datum given to them by the teachers of this science is erroneous, it behoves these teachers not to let an erroneous datum pass current: they are called on to settle this vexed question one way or other. The times give proof. The work of Mr. Benjamin Kidd on Social Evolution, which has been so much lauded, takes Weismannism as one of its data; and if Weismannism be untrue, the conclusions Mr. Kidd draws must be in large measure erroneous and may prove mischievous.
Postscript.—Since the foregoing pages have been put in type there has appeared in Natural Science for September, an abstract of certain parts of a pamphlet by Professor Oscar Hertwig, setting forth facts directly bearing on Professor Weismann's doctrine respecting the distinction between reproductive cells and somatic cells. In The Principles of Biology, § 77, I contended that reproductive cells differ from other cells composing the organism, only in being unspecialized. And in support of the hypothesis that tissue-cells in general have a reproductive potentiality, I instanced the cases of the Begonia phyllomaniaca and Malaxis paludosa. In the thirty years which have since elapsed, many facts of like significance have been brought to light, and various of these are given by Professor Hertwig. Here are some of them:—
"Galls are produced under the stimulus of the insect almost anywhere on the surface of a plant. Yet in most cases these galls, in a sense grown at random on the surface of a plant, when placed in damp earth will give rise to a young plant. In the hydroid Tubularia mesembryanthemum, when the polyp heads are cut off, new heads arise. But if both head and root be cut off, and the upper end be inserted in the mud, then from the original upper end not head-polyps but root filaments will arise, while from the original lower end not root filaments but head-polyps will grow.... Driesch, by separating the first two and the first four segmentation spheres of an Echinus ovum, obtained two or four normal plutei, respectively one half and a quarter of the normal size.... So, also, in the case of Amphioxus, Wilson obtained a normal, but proportionately diminished embryo with complete nervous system from a separated sphere of a two- or four- or eight celled stage.... Chabry obtained normal embryos in cases where some of the segmentation-spheres had been artificially destroyed."
These evidences, furnished by independent observers, unite in showing, firstly, that all the multiplying cells of the developing embryo are alike; and, secondly, that the soma-cells of the adult severally retain, in a latent form, all the powers of the original embryo-cell. If these facts do not disprove absolutely Professor Weismann's hypothesis, we may wonderingly ask what facts would disprove it?
Since Hertwig holds that all the cells forming an organism of any species primarily consist of the same components, I at first thought that his hypothesis was identical with my own hypothesis of "physiological units," or, as I would now call them, constitutional units. It seems otherwise, however; for he thinks that each cell contains "only those material particles which are bearers of cell-properties," and that organs "are the functions of cell-complexes." To this it may be replied that the ability to form the appropriate cell-complexes, itself depends upon the constitutional units contained in the cells.
APPENDIX C.
THE INHERITANCE OF FUNCTIONALLY-WROUGHT MODIFICATIONS: A SUMMARY.
The assertion that changes of structure caused by changes of function are transmitted to descendants is continually met by the question—Where is the evidence? When some facts are assigned in proof, they are pooh-poohed as insufficient. If after a time the question is raised afresh and other facts are named, there is a like supercilious treatment of them. Successively rejected in this way, the evidences do not accumulate in the minds of opponents; and hence produce little or no effect. When they are brought together, however, it turns out that they are numerous and weighty. We will group them into negative and positive.
* * * * *
Negative evidence is furnished by those cases in which traits otherwise inexplicable are explained if the structural effects of use and disuse are transmitted. In the foregoing chapters and appendices three have been given.