No. 550] NOTES AND LITERATURE 



evidence that the supposed creature did exist. And what is the 

 evidence? Why, the observed facts of arachnid structure, ex- 

 actly these and no others, else the ancestor would not be held as 

 hypothetical. No hypothesis can, of itself, add any new facts. 

 The hypothetical ancestor has, consequently, done nothing for 

 the case except to disguise the obvious difficulty there is in seeing 

 how an actual arachnid can be transformed into an actual verte- 



The only really safe rule for using hypotheses in biology is 

 that no hypothesis shall be made except to help toward answer- 

 ing a question by formulating a clear provisional answer to 

 that question. The making of hypotheses and using them before 

 they are themselves proved for the solution of other problems 

 than those to which they immediately pertain, is perilous busi- 

 ness. I may without danger, even with profit, construct an 

 imaginary enteropneust to aid my efforts to answer the question 

 of how the enteropneustic branchial apparatus or the organ in 

 this group called a notochord, arose phylogenetically. But be- 

 fore this imaginary creature can do me real service I must es- 

 tablish at least a strong probability that such an imaginary ani- 

 mal once actually existed. Nor must I fail to notice that such 

 probability can be established only by bringing new evidence 

 into the case. The facts on which I based the hypothetical ani- 

 mal can not be used over again to prove the reality of that ani- 

 mal. But if I must be thus cautious in resorting to an hypothet- 

 ical enteropneust for the interpretation of actual enteropneusts, 

 how much more need would there be for caution should I venture 

 to invoke an hypothetical enteropneust for interpreting actual 

 vertebrates ! 



Reflections of this nature opened my eyes several years ago 

 when I was struggling with the enteropneust hypothesis of verte- 

 brate descent, to the very slight chance there is of ever solving 

 the problem of vertebral origin. 



Do I then regard all hypotheses on the problem as equal in 

 value because all alike are equally futile ? By no means. I con- 

 sider each one of them to be of real worth, particularly those 

 that have been worked out as have Patten's arachnid hypothesis, 

 Gaskell's crustacean hypothesis, the annelid hypothesis and the 

 enteropneust hypothesis. They are of worth because each shows 

 quite clearly certain possible developmental courses that may 

 have been followed in the origin and progress of the great verte- 

 bral stock. To be explicit, it seems to me that Patten has shown 



