HEREDITY AND VARIATION 



definite loci in the chromosomes. There is no evidence of that " blend- 

 ing inheritance" which pre-Mendelian biolo 1 to be the 

 rule. Although • cytoplasmic inheritam anno! be ruled oul 

 non-existent — indeed, there is evidence thai in some instan< ea it m 

 important— it is probable that the part that it plays in heredity ia sub- 

 ordinate to the action of the nuclear genes. It should be mentioned tl 

 while, for technical reasons, Mendelian inheritance has been studi 

 chiefly in higher organisms, it has been observed also anion- M 

 Fungi and Algae (including unicellular types such as Chlamydomonas). 



(2) Hybridisation followed by segregation has been an important 

 source of new forms (re-combinations). Hybridisation experiments, 

 again for technical reasons, are commonly carried out with ra 

 differing in comparatively few genes ; but what is known about 

 " wide crosses " (between widely differing species or between distinct 

 genera) suggests that the observed divergences from straightforward 

 Mendelian segregation are due partly to the large number of ge; 

 involved, and partly to secondary complications such as polyploidy, 

 chromosome re-arrangement and indirect effects of cytoplasmic 

 differences. , 



Mutation is the basis of heritable variation and thus of evolution. 

 " Heredity is essentially a conservative process. Evolution is possible 

 only because heredity is counteracted by another force opposite in 

 effect, namely mutation " (Dobzhansky). Strictly, the term mutation 

 should be applied only to gene-mutation, i.e. to a change in the con- 

 stitution of a gene ; but it is often more convenient to use the term 

 in a wider sense so as to include such changes as chromosome-rearran 

 ment and polyploidy, which may likewise give rise to heritable varia- 

 tions. 



Nothing definite can be predicated as yet regarding the rate of origin 

 of new species by gene-mutation alone. It is on the other hand clear 

 that new species can arise by a sudden or " cataclysmic " method, 

 viz., through polyploidy following upon hybridisation. So far, poly- 

 ploidy has been observed only within certain groups of organisms, 

 mainly among plants ; but within these limits there can be little doubt 

 as to its importance as a subsidiary method of specie- production. 



The persistence of a new form, however produced, depends 0- 



tially on the action of some mechanism of isolation, which prevents 

 the new type from disappearing through free inter-crossing with pre- 

 existing types. Geographical separation and any structural featui 

 or physiological conditions (cross-incompatibility or hybrid-sterility) 

 that prevent fertile union between different species, provide such 



