1899] REGENERATION 325 



and this position I still maintain. Previously to this, however, I had 

 come to the conclusion, based upon the theory of the reducing division, 

 that a corresponding process of reduction must take place in the male 

 cell, and some time later this was actually demonstrated by Oscar 

 Hertwig. It is well known that during the succeeding years many 

 excellent observers have carried on this difficult investigation on the 

 processes of reduction in male and female germ-cells, and I need only 

 mention the names of Boveri, Henking, vom Eath, Euckert, Hacker, 

 Korschelt, Ischikawa, and van der Stricht, and in the domain of botany, 

 Strasburger, Ischikawa, Calkins, Belajeff, and Guignard. Thus the 

 conclusion has been gradually more and more firmly established that 

 in the sex-cells of animals, as well as in those of plants, a reduction of 

 the number of chromosomes takes place, and that in the majority of 

 carefully investigated cases this process is brought about in the same 

 manner, that is, by a peculiar form of nuclear division which occurs 

 only in this connection. Eecently, too, increasingly reliable evidence 

 has been furnished that a similar reducing nuclear division precedes 

 conjugation in unicellular plants and animals (Maupas, E. Hertwig, 

 Schaudinn, and others). 1 Isolated observations are always cropping 

 up which seem to necessitate different interpretations ; but we may 

 expect, as has often happened before, that more thorough investigations 

 will succeed in removing the discrepancies in some way or other. A 

 review of the facts now established cannot but convince us that 

 for a large series of organisms the reduction -hypothesis is already 

 proved. 



This advance in our knowledge was only possible on the basis of 

 changing theories, one growing out of another, and it would be absurd 

 to object to such modifications. It seems to me, on the contrary, that 

 the great value of hypotheses and theories is just that they do bring 

 about the necessity of modifying them. They form the indispensable 

 ladder by which investigation descends step by step into the depths of 

 the biological mine until it comes upon a new vein of ore — fresh guid- 

 ing facts — when it pauses a while, exploring and exploiting them in all 

 directions, and finally by proving them secures a basis of operations 

 from which a new ladder may be sunk to lower depths. 



One of my critics has compared my " theories " to " towns in the 

 Far West," the houses of which are barely erected when they are 

 taken down again to be rebuilt farther out in the unknown land. I 

 accept the simile, provided it be not forgotten that the first house of 

 the advancing pioneer must remain standing and in use for a time 

 before the region beyond becomes accessible to further colonisation. 



I admit that I have not only made unavoidable mistakes, but also 

 some which might perhaps have been avoided, and that I have 



1 Cf. the excellent review and critical summary of facts recently given by Hacker ( Verh. 

 Deutsche zool. Gesellsch. Jena, 1898) under the title " Uber vorbereitende Theilnngsvorgange 

 bei Thieren und Pflanzen," p. 95. 



