No. 607] 



RATS AND EVOLUTION 



find out the causes for genovariability. Real mutation, 

 as far as we know, exclusively consists of an occasional 

 loss of a gene without visible cause. Mutation therefore 

 can at the utmost heighten the potential variability by 

 one, De Vries's conception of periods of mutation is at 

 present only of historical interest. 



In our opinion, crossing, recombination of genes by 

 mating of individuals of unequal genotype, is to be re- 

 garded as the only real cause of variability. There is no 

 good reason to change the opinion of one of us, namely, 

 that there exist three different kinds of variability.^ 



A. Modification, the non-inheritable effect of the non- 

 genetic developmental factors. 



B. Real inheritable variation caused by mutation, loss 

 of genes. 



C. Real inheritable variation by reeoml>ination of 

 genes. 



Lotsyhas subscribed to our statement {loc.cit.) with the 

 exception that he denies the existence of loss-mutations. 



We can no more say that species originate by crossing, 

 than, that they originate by isolation. New pure lines of 

 autogamous plants, the kind of species for which Lotsy 

 wants to reserve the term, can of course originate in the 

 descendance of one hybrid plant. There is no funda- 

 mental difference between evolution in these plants in a 

 state of cultivation and what it must be in nature. But 

 in allogamous organisms, we will only in exceptional cases 

 meet in nature the same course of evolution as in our 

 cages or experimental plots. 



Even if crossing in the widest sense is the sole cause of 

 variabilty, we must not suppose that, as a rule, new species 

 come into being in the F2 or generation from a cross. 

 If we make a hybrid between s])e(Me>. tliis hybrid indi- 

 vidual will have a total potential variability which is at 

 least as great as the number of genes whit^li were not 

 common property of both the foi ius ( i-.k-simI. If we com- 



the Inheritable Characters," Rous' serie \ CrtriiK.' mi,! Autsitze iiber Ent- 

 wicklungsmechanik, Leipzig, 1911. 



