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. [150] Certain English authors have applied the term “modification” to all kinds of organic properties acquired from without, whether they are adapted or not. 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.