I.
Just now there is much controversy in regard to the factors of evolution. Both Darwin and Spencer, the two greatest expounders of the modern theory of evolution, acknowledge and insist upon at least four factors; viz. the two Lamarckian and the two distinctively Darwinian. The only difference between them is in the relative importance of the two sets; Spencer regarding the former and Darwin the latter as the more potent. But some late Darwinians have gone far beyond Darwin himself in their estimate of the power of the most distinctive Darwinian factor, viz. natural selection. Weismann and Wallace have each written a book, and Lankester many excellent articles to show that natural selection is the one sole and sufficient cause of evolution, that changes during the individual life whether by effect of the environment or by the use and disuse of organs are not inherited at all, that Lamarck was wholly wrong and that Darwin (in connection with Wallace) is the sole founder of the true theory of evolution, and finally that Darwin himself was wrong only in making any terms whatever with Lamarck.
The argument for this view has, perhaps, been most strongly put by Weismann and is based partly on experiments, but mainly on his ingenious and now celebrated theory of the immortality of germ-plasm. The animal body consists of two kinds of cells wholly different in function, somatic cells and germ-cells, including in this last the sexual elements both male and female. Somatic cells are modified and specialised for the various functions of the body; germ-cells are wholly unmodified. The somatic cells are for the conservation of the individual life, germ-cells for the conservation of the species. Now according to Weismann, inheritance is only through germ-cells. Environment affects only the somatic cells and therefore changes produced by environment cannot be inherited. Sexual generation was introduced for the purpose of producing variability in progeny and thus furnishing material for natural selection, as this was the only means of evolutionary advance. Weismann made many experiments on animals, especially by mutilation, to show that somatic changes are not inherited.
We shall not argue this question but content ourselves with making three brief remarks.
1. If the views presented in this article be true, then the Lamarckian factors must be true factors, because there was a time when there were no others. They were necessary therefore to start the process of evolution, even if no longer necessary at present.
2. But if the Lamarckian factors were ever operative, they must be so still, though possibly in a subordinate degree. A lower factor is not abolished, but only becomes subordinate to a higher factor when the latter is introduced. Thus it may well be that Lamarckian factors are comparatively feeble at the present time and among present species, especially of the higher animals, and yet not absent altogether. In the earliest stages of evolution there was a complete identification of germ-cells and somatic cells—of the individual with the species. In such cases, of course, the effect of environment must be inherited and increased from generation to generation. But the differentiation of germ and somatic cells was not all at once; it was a gradual process, and therefore the effect of the environment on the germ-cells through the somatic cells must have continued, though in decreasing degree, and still continues. The differentiation is now, in the higher animals, so complete that germ-cells are probably not at all affected by changes in somatic cells, unless these changes are long continued in the same direction and are not antagonised by natural selection.
3. It is a general principle of evolution that the law of the whole is repeated with modifications, in the part. This is a necessary consequence of the Unity of Nature. We ought to expect therefore and do find, that the order of the use of the factors of evolution is the same in the evolution of the organic kingdom, in the evolution of each species, and in the evolution of each individual. In all these the physical factors are first powerfully operative, then become subordinate to organic factors, and these in their turn to psychical and rational factors. Therefore, as the individual in its early stages, i. e. in embryo and infancy, is peculiarly plastic under the influence of the physical environment and afterwards becomes more and more independent of these: so a species when first formed is more plastic under the influence of the Lamarckian factors and afterwards becomes more rigid to the same. And so also the organic kingdom was doubtless at first more plastic under Lamarckian factors, and has become less so in the present species, especially of the higher animals. The principal reason for this, as we have already seen, is the increasing differentiation of germ and somatic cells, and the removal of the former to the interior where they are more and more protected from external influence.