84 PEOCEEmKGS OF THE 



absolutely pure form. Xo better experiment could, to mv 

 mind, be made to prove the constancy ot: species than tins 

 involuntary one. 



Now, after all this evidence for the constancy of species and 

 their origin by crossing, the question remains : 



What is the Vera Causa of the Change ]jroduced by Crossing? 



I think we can safely say that it is the introduction of a new 

 factor, and that by this the problem of tlie origin of species is 

 brought into line Avith the problem of the origin of chemical 

 compounds. In species it is the new factor, in chemical 

 compounds the new element introduced, which causes the change. 



Just as the introduction of a new element may cause a great or 

 small effect, dependent on the kinds of elements already present 

 and the kind of element introduced, the new factor may cause 

 a small or a great change. 



Not the size of the jump is essential : the essential thing is 

 that the introduction of the new factor causes a definite change, 

 so that no intermediate stages between two species difierent in 

 oue factor occur. From this it might seem that my theory 

 requires as many links as Darwin's does. Tet I do not think so. 

 Considerable jumps and small jumps are possible, according to my 

 views. There are among organisms differences of a major and 

 differences of a minor degree. 80, for example, all vertebrates are, 

 as far as their skeleton is concerned, built very much on the same 

 plan. So are Augiosperms in respect to their flower structure. 

 Now such ground-[:lans, I take it, are caused by the introduction 

 of a major genetic factor, and lesser changes in the ground-plan, 

 like the transformation of a foot to the fin of a whale, being due 

 to minor genetic factors. 



I may illustrate this by a chemical analogy. The introduction 

 of Nitrogen in the Benzol ring 



/CH\^ ^CH^ 



CH CH CH CH 



II I changes this to Pvridin |l j 



CH CH ' CH CH 



\cH y^ \ N ^ 



and from this Pyridin all genuine Alkaloids are derived. So the 

 introduction of the unknown factor .r may have caused in some 

 invertebrate the formation of the skeleton, and, if this be true, it 

 is hopeless to loolt for a transitional form between an invertebrate 

 and a vertebrate, as none such ever existed. The vertebrates then 

 arose by a cross of two invertebrates, who together possessed the 

 set of factors necessary for the formation of a skeleton, while 

 each of them alone lacked one or more factors of it. 



This brings me to the first origin of species. I think living 

 organisms of different kiud, e.g., possessing different sets of 



