But what is known about the inheritance of those properties which beyond any doubt may be said to have originated in the adult individual as such, and of which lesions and adaptations proper, as shown for instance among amphibious plants, are instances of the two most typical groups?[150] Weismann did good service by putting an end to the scientific credulity which prevailed with regard to this subject. Weismann was led by his theory of the germ plasm to deny the inheritance of acquired characters of the typical kinds. He could not imagine how the effect of any agent upon the adult, be it of the merely passive or of the adaptive kind, could have such an influence upon the germ as to force it to produce the same effect in spite of the absence of that agent. In fact, that is what the inheritance of acquired characters would render necessary, and a very strange phenomenon it would be, no doubt. But, of course, taken alone, it could never be a decisive argument against such inheritance. I fully agree, that science is obliged to explain new facts by what is known already, as long as it is possible; but if it is no longer possible, the theory of course has to be changed, and not the facts. On this principle one would not neglect the fact of an inheritance of acquired properties, but on the contrary one perhaps might use it as a new evidence of vitalism.
But are there any facts?
At this point we come to speak about the second group of Weismann’s reasonings. He not only saw the difficulty of understanding inheritance of acquired characters on the principles of the science of his time, but he also criticised the supposed facts; and scarcely any of them stood the test of his criticism. Indeed, it must fairly be granted that not one case is known which really proves the inheritance of acquired characters, and that injuries certainly are never found to be inherited. In spite of that, I do not believe that we are entitled to deny the possibility of the inheritance of a certain group of acquired characters in an absolute and dogmatic manner, for there are a few facts which seem at least to tend in the direction of such an inheritance, and which seem to show that it might be discovered perhaps one day, if the experimental conditions were changed.
I am not referring here to the few cases in which bacteria were made colourless or non-virulent by outside factors, or in which certain fungi were forced to permanent agamic reproduction by abnormal external conditions and were shown to retain their “acquired properties” after the external conditions had been restored. In these cases only reproduction by simple division occurred, and that does not imply the true problem of inheritance. Nor am I referring to the few cases of non-adaptive “modifications” found by Standfuss and Fischer, in which butterflies that had assumed an abnormal kind of pigmentation under the influence of abnormal temperature acting upon the pupa, were seen to form this same kind of pigmentation in the next generation under normal conditions of temperature. These cases, though important in themselves, are capable perhaps of a rather simple explanation, as in fact has been suggested. Some necessary means both of inheritance and of morphogenesis, the former being present in the propagation cells, may be said to have been changed or destroyed by heat, and therefore, what seems to be inherited after the change of the body only, would actually be the effect of a direct influence of the temperature upon the germ itself.[151] Let me be clearly understood: I do not say that it is so, but it may be so. What seems to me to be more important than everything and to have a direct bearing on the real discovery of the inheritance of acquired characters in the future, is this. In some instances plants which had been forced from without to undergo certain typical morphological adaptations, or at least changes through many generations, though they did not keep the acquired characters permanently in spite of the conditions being changed to another type, were yet found to lose the acquired adaptations not suddenly but only in the course of three or more generations. A certain fern, Adiantum, is known to assume a very typical modification of form and structure, if grown on serpentine; now Sadebeck,[152] while cultivating this serpentine modification of Adiantum on ordinary ground, found that the first generation grown in the ordinary conditions loses only a little of its typical serpentine character, and that the next generation loses a little more, so that it is not before the fifth generation that all the characters of the serpentine modification have disappeared. There are a few more cases of a similar type relating to plants grown in the plains or on the mountains. There also it was found to take time, or rather to take the course of several generations, until what was required by the new conditions was reached. Of course these cases are very very few compared with those in which a sudden change of the adaptive character, corresponding to the actual conditions, sets in; but it is enough that they do exist.
Would it not be possible at least that adaptations which last for thousands of generations or more might in fact change the adaptive character into a congenital one? Then we not only should have inheritance of acquired characters, but should have a sort of explanation at the same time for the remarkable fact that certain histological structures of a very adapted kind are formed ontogenetically before any function exists, as is known to be the case with the structures in the bones of vertebrates, for instance. Experiments are going on at Paris, and perhaps in other places of scientific research also, which, it is hoped, will show that animals reared in absolute darkness for many generations will lose their perfectly formed eyes, and that animals from the dark with very rudimentary eyes will be endowed with properly functioning ones, after they have been reared in the light for generations. Such a result indeed would account for the many animals, of the most different groups, which live in dark caves and possess only rudiments of eyes: functional adaptation is no longer necessary, so-called atrophy by inactivity sets in, and the results “acquired” by it are inherited.[153]
But enough of possibilities. Let us be content at present to know at least a few real instances with regard to the slowness of the process of what might be said to be “re-adaptation” in some plants. This process shows us a way by which our problem may some day be solved; it allows us to introduce inheritance of acquired characters as a legitimate hypothesis at least, which not only will explain many of the diversities in systematics historically, but also can be called, though not a causa vera, yet certainly more than a mere fiction.
OTHER PRINCIPLES WANTED
We have only dealt with the probability of the inheritance of morphological or physiological[154] adaptation. If that could really be considered as one of the factors concerned in the theory of descent, many, if not all of those congenital diversities among organic species which are of the type of a true structural correspondence to their future functional life, might be regarded as explained, that is, as reduced to one and the same principle. But nothing more than an explanation of this kind of diversities is effected by our principle, and very much more remains to be done, for organic diversities not only consist in specifications and differences as to histology, but are to a much more important degree, differences of organisation proper, that is, of the arrangement of parts, in the widest sense of the word.[155]