PRESENT CRISIS IN EVOLUTIONARY THOUGHT 17 



rence of factorial mutants, while Dr. Albert F. Blakeslee has 

 demonstrated the existence of chromosomal mutants. When 

 unqualified, the term mutant usually denotes the factorial 

 mutant, which arises from a change in one or more of the 

 concatenated genes (hereditary factors) of a single chromo- 

 some (nuclear thread) in the germinal {i.e. gametic) complex. 

 All such changes are called factorial mutations. They are 

 hereditarily transmissible, and affect the somatic characters 

 of the race permanently, although, in rare cases, such as that 

 of the bar-eyed Drosophila mutant, the phenomenon of rever- 

 sion has been observed. The chromosomal mutant, on the 

 contrary, is not due to changes in the single factors or genes, 

 but to duplication of one or more entire chromosomes (linkage- 

 groups) in the gametic complex. Like the factorial mutant, 

 it produces a permanent and heritable modification. The in- 

 crease in nuclear material involved in chromosomal mutation 

 {i.e. duplication) seems to cause a proportionate increase in 

 the cytoplasmic mass of the single somatic cells, which mani- 

 fests itself in the phenotype as giantism. De Vries' (Enothera 

 gigas is a chromosomal mutant illustrative of this phenome- 

 non. Besides the foregoing, there is the pseudomutant pro- 

 duced by the factorial recombination, which results from a 

 crossover, i.e. an exchange of genes or factors between two 

 germinal chromosomes of the same synaptic pair. This recip- 

 rocal transfer of genes from one homologous chromosome to 

 another happens, in a certain percentage of cases, during synap- 

 sis. The percentage can be artificially increased by exposing 

 young female hybrids to special conditions of temperature. 



If these new mutant forms could be regarded as genuine new 

 species, then the fact that such variations are heritable and 

 come within the range of actual observation, would constitute 

 the long-sought empirical proof of the reality of evolution. 

 Consciously or subconsciously, however, De Vries recognized 

 that this was not the case; for he refers to mutants as ''ele- 

 mentary species," and does not venture to present them as 

 authentic organic species. 



