THE PHENOMENA OF MUTATION 185 



the conclusion that the stouter character of the 

 organs in this plant is causally connected with the 

 increased number of chromosomes. Where the 

 number of cells formed is approximately similar, as 

 in two allied forms of plant in this case, the greater 

 size of the cells would naturally give a stouter habit, 

 but it is clear that large cells do not necessarily 

 mean greater size. The cells of Salamander and 

 Proteus are the largest found among Vertebrates, 

 but those Amphibia are not the largest Vertebrates. 

 It is curious to note how different are these dis- 

 coveries concerning differences in the number of 

 chromosomes from the conception of Morgan that 

 a mutation depends on a factor situated in a part 

 of one chromosome. 



More copious details concerning mutations will 

 be found in the publications cited. The question 

 to be considered here is how far the claim is justified 

 that the facts of this kind hitherto discovered afford 

 an explanation of the process of evolution. It 

 seems probable that mutations are of different kinds, 

 as exemplified in (Enoihera by gigas and rubricalyx 

 respectively, the former producing only sterile 

 hybrids, the latter behaving exactly like a Mendelian 

 unit. There can be little doubt that, as Bateson 

 states, numerous forms recognised as species or 

 varieties in nature differ in the same way as the 

 races or breeds of cultivated organisms which differ 

 by factors independently inherited. There are facts, 

 however, which prove that all species are not sterile 

 inter se, and that their characters when they are 

 hybridised do not always segregate in Mendelian 

 fashion. John C. Phillips, 1 for example, crossed 



1 Journ. Exper. ZooL, vol. xviii., 1915, 



