THE INDIVIDUAL AND THE SPECIES 199 



himself what may be meant by this behaviour of the 

 germ cells, and he will certainly see that several inter- 

 pretations are possible. But suppose that the chro- 

 matin consists of an incredibly large number of bodies 

 differing in chemical structure from each other, and 

 occupying definite positions with regard to each 

 other ; and suppose that there is a mechanism of 

 unimaginable complexity in the cell capable of rejecting 

 some of these chemically individualised parts, and 

 of " assembling " or arranging the others in much the 

 same way as an engineer assembles the parts of a 

 dynamo when he completes the machine. Then we 

 may regard the hypothetical discrete bodies which 

 form the hypothetical nuclear architecture as the 

 material carriers of Mendelian characters. It is 

 strange that the correspondence of such a logically 

 constructed mechanism with the effects which it would 

 produce if it existed should be regarded as a proof 

 that it does exist, yet biological speculation has 

 actually made use of such an argument. ' It seems 

 exceedingly unlikely that a mechanism so exactly 

 adapted to bring it " (the separation from each other 

 of the Mendelian material " factors " of inheritance) 

 " about should be found in every developing germ 

 cell if it had no connection with the segregation of 

 characters that is observed in experimental breeding." 

 Put quite plainly this argument is as follows : there 

 is a certain segregation to be seen in experimental 

 breeding, and certain processes may be observed to 

 occur in the developing germ cell. Add to these 

 processes many others logically conceivable, and add 

 to the observed material structure of the cell another 

 structure also logically conceivable. Then the assumed 

 mechanism and structure is " exactly adapted ' 

 to produce the effects which are to be explained. 



