66 PROBLEMS OF EVOLUTION 



A seeming Here I must briefly anticipate a difficulty with which I shall 

 concession deal ;n thg chapter Qn N atura i Selection. Though external 



conditions cannot cause a particular variation, yet they may 

 stimulate variability. What happens I imagine to be this. 

 Natural Selection is constantly strengthening heredity. As long 

 as the same characteristics continue to be selected for survival, 

 that is, as long as the conditions remain the same, the species 

 attains greater and greater fixity. The deviations from the 

 normal become smaller and smaller. But, when there is a 

 change of conditions, variability shows itself. The variations 

 are not necessarily adaptations ; more frequently they bear no 

 relation to the new circumstances. The change that appears 

 in wild ducks after several generations of domestication is a 

 well known instance of this. The white collar round the neck 

 of the Mallard becomes much broader and more irregular and 

 white feathers appear in the duckling's wings. 1 



Sometimes variations induced by change of environment be- 

 come constant, though Natural Selection, as far as we can see, 

 does not intervene to stereotype them, and here we enter upon 

 one of the most difficult parts of our subject. 2 I shall have to 

 refer to it again. At present, it will be enough to say that 

 Lamarckism derives no support from the facts. There is merely 

 a shift to another position (already possible to the species but 

 previously ruled out by stringent Natural Selection, or some 

 other influence but little understood), or else a failure of 

 heredity, a failure to build up to completion all that has 

 become evolved in the course of the phylogeny. For the 

 Neo-Darwinian the only difficulty which the facts present is 

 the apparent existence of stability apart from the operation of 

 Natural Selection. 



Summary I have tried to show that it is very difficult even to imagine 

 the means by which acquired characters might be transferred 

 from a bodily organ to the germ-plasm : that breeders hardly 



1 Mr Hewitt, quoted by Darwin, Animals and Plants under Domestication, vol. ii. 

 p. 263. 



2 See Prof. Adam Sedgwick's address, Brit. Association. 1899 (Nature, Sept. zi) ; 

 also Prof. Mark Baldwin's letter in Nature, Oct. 19, 1899. 



