266 PHYSICAL BASIS OF HEEEDITY 



that kill the germ-cells — gametes — before they are ready 

 for fertilization, has already been invoked by De Vries 

 and others to explain the peculiarity of '^double recipro- 

 cal hybrids.'' 



Is THE Direction of Mutation Given in the Constitution 



OF THE Genes? 



When writers have brought forward evidence of con- 

 tinuous and progressive change in a character, they have 

 not concerned themselves with the analysis of the change 

 in the germ-plasm that has brought it about — in fact, in 

 most of these cases the possibility of advance in a princi- 

 pal gene or of advance through modifying genes has not 

 been appreciated or even understood. Paleontologists 

 who have in the main been the strong advocates of ortho- 

 genesis have based their conclusions on the observed 

 advances in a character in the same series and in ^^ paral- 

 lel'' series. They overlook the fact that to-day there is 

 experimental evidence demonstrating that variations as 

 small even as those they record have been shown to rest 

 on mutational stages. If the progress has been in the 

 direction of adaptation, natural selection of small mutant 

 differences will completely cover their findings. If it is 

 claimed that in some of these cases the orthogenetic series 

 is not in the line of adaptive advance, the burden of proof 

 lies heavily on their shoulders. Moreover, the fact, that 

 recent work has made clear, that genes generally have 

 more than a single effect on the organization, opens wide 

 the door of suspicion, for the observed morphological 

 progress might be a by-product of influences that have 

 other and important, though unseen or unknoAvn, effects. 

 In a word, an orthogenetic series of changes does not in 

 itself without a closer analysis than has as yet been fur- 

 nished, establish that an innate principle, urge, vis-a-tergo, 

 ''kick/' or vital ''force'' is causing the successive moves. 

 The genetic evidence concerning multiple factors must 

 create at least a strong suspicion against the '^will to 



