PRESENT CRISIS IN EVOLUTIONARY THOUGHT 23 



tetraplodd race fulfills the sterility test of a distinct species. 

 Whether or not it fulfills the endurance test of survival under 

 natural condition is doubtful, inasmuch as diploid Daturas 

 are about three times as prolific as the tetraploid race. More- 

 over, as Blakeslee himself confessed in a lecture at Woods 

 Hole attended by the present writer in the summer of 1923, 

 the origin of a balanced tetraploid form from the normal 

 diploid type by simultaneous duplication of all the chromo- 

 somes in the diploid complex, is an event that has yet to be 

 witnessed. Nor is any gradual transition from the diploid 

 to the tetraploid race, by way of unbalanced types and tri- 

 ploids, conceivable, seeing that such forms are too sterile 

 to maintain themselves, and are, in fact, incapable of trans- 

 mitting their own type in the absence of artificial interven- 

 tion. There are, it is true, some instances, in which diploid 

 and tetraploid races and species occur together in cultivation 

 and in nature. In certain cases, this tetraploidy is merely 

 apparent, being due to fragmentation of the chromosomes; in 

 other cases, it is really due to chromosomal duplication, giving 

 rise to genuine tetraploid forms. The question is often hard 

 to decide, the mere number of the chromosomes being not, 

 in itself, a safe criterion. Of the actual origin, however, of 

 tetraploid from diploid races we have as yet no observational 

 evidence. Hence Blakeslee's researches on the chromosomal 

 mutant have so far failed to furnish experimental proof of 

 the origin of a genuine new species. Besides, waiving all other 

 considerations, the limits within which chromosomal duplica- 

 tion is possible are of necessity sd narrow, that, at best, this 

 phenomenon can only be invoked to explain a very small 

 range of variation. In fact, it is doubtful whether haploidy, 

 triploidy, and tetraploidy have any important bearing what- 

 ever upon the problem of the origin of species. (See Addenda.) 

 The mutation, then, in so far as we have experimental 

 knowledge of it, does not fulfill requirements of a specific 

 change. It cannot even be regarded as an elementary step 

 in the direction of such a change. With this admission, De- 



