64 THE EVOLUTION THEORY 



that the germ-plasm is without primary constituents, and suppose 

 that, after each functional variation of a part, telegraphic notice 

 is sent to the germ-plasm b}^ way of the brain as to how it has to 

 alter its ' physico-chemical constitution,' so that the descendants may 

 receive some benefit from the accjuired improvement? 



I am not of the number of those who believe that we already 

 know all, or at least nearly all, that is essential, but am rather 

 convinced that whole regions of phenomena are still sealed to us, 

 and I consider it probable that the nervous system in particulai* is 

 not yet exhaustively known to us, either in regard to its functioning 

 or in regard to its jfinest structural architecture, although I gratefully 

 recognize the advances in this domain that the last decades have 

 brought about. In any case, such assumptions as I have just 

 indicated, or similar ones, seem to me quite too improbable to furnish 

 any foothold for progress. Yet we must always remain conscious 

 that we cannot decide as to the possibility or impossibility of any 

 biological process whatever from a purely theoretical standpoint, 

 because we can only guess at, not discern, the fundamental nature 

 of biological processes. At the close of this lecture I shall return 

 to the question of the theoretical conceivability of an inheritance of 

 functional adaptations ; but first of all we must consider the facts 

 and be guided by them alone. If they prove, or even make it seem 

 probable, that such inheritance exists, then it must be possible, and 

 our task is no longer to deny it, but to find out how it can come about. 



Let us therefore investigate the question whether an inheritance 

 of acquired characters, that is, in the first place, of functional adapta- 

 tions, is demonstrable from experience. We shall speak later on of the 

 effect of climatic and similar influences in causing variation ; the case 

 in regard to them is quite difierent, because they undoubtedly affect 

 not only the parts of the body but the germ-cells as well. 



When we inquire into the facts which have been brought forward 

 by the modern adherents of the Lamarckian principle as proofs of 

 the inheritance of acquired characters in this restricted sense, we 

 shall find that none of them can withstand criticism. 



First, there are the numerous reputed cases of the inheritance 

 of mutilations and losses of whole parts of the body. 



It is not without interest to note here how opinion in regard 

 to this point has altered in the course of the debate. 



At the beginning of the discussion they were all brought forward 

 as evidence of undoubted value for the Lamarckian principle. 



At the Naturalists' Congress in Wiesbaden in 1887, kittens with 

 only stumps of tails were exhibited, and they were said to have 



