THE PROBLEMS PRESENTED 27 



de Vries. It is well to enumerate here the six different factors in 

 organic evolution which might claim a share in the production of 

 such humble phenomena as form the subject-matter of this volume — 

 they are : 



1. Personal Selection of Darwin. 



2. Sexual Selection. 



3. Histonal or Cellular Selection of Roux. 



4. Germinal Selection. 



5. Inheritance according to Mendelian principles. 



6. Inheritance of Mutations. 



There is a somewhat severe and ill-defined condition attached 

 to the formula in question for it demands that such modifications 

 as will satisfy the neo-Darwinians shall not be correlated with 

 any useful character. 1 If such a conditio sine qua non were taken 

 too literally it would at once foreclose the case as to the possibility 

 of transmission of modifications at all, the questions of issue ought 

 in that case never to have been raised — and, cadit qucestio. This 

 cannot be the intention of the biologist who propounds the formula. 

 It could not reasonably be carried so far as to insist that a modifica- 

 tion arising from a certain habit, active or passive, in an animal, 

 and which on that account, and on paper, may loosely be said to be 

 ' correlated ' with it, is to be ruled out. That would be tantamount 

 to saying for example, that, because an animal must lie down in a 

 certain attitude when it rests, or walk or run in a certain manner, 

 in other words that it is useful to exist, certain modifications claimed 

 to be due to these fundamental parts of existence must be excluded 

 from the inquiry. The neo-Darwinian is not a critic easy to be 

 entreated, but that he would not claim. Let me take one example 

 of what I mean. A short-haired dog will spend a considerable 

 part of its daily life, and presumably a long line of ancestors did 

 so too, lying with its forelegs planted in front of its chest and its 

 head either raised in the air when awake or resting on the upper 

 surface of the forelegs (of course the familiar attitude of a dog with 

 its body and head curled up and fore-legs doubled is not referred 

 to here). If the hairy coat be examined over its neck and jaw, 

 which lie in this attitude, on and against the forelegs, a remarkable 

 reversal of the direction of the hairs is found and the outline of 

 this forms an accurate mould of the surface applied to the forelegs. 

 This is transmitted of course from previous generations of domestic 

 dogs. A precisely analogous reversal of the hairs is found on the 

 under or extensor surfaces of the forelegs, matching with wonderful 



1 My italics. 



