382 ANIMAL BIOLOGY 



ciation of its limitations has but accentuated its possibilities. 

 Natural selection may automatically act as a 'sieve' and sort 

 out the new combinations and mutations presented — retain the 

 fit and discard the unfit — and so afford a natural explanation of 

 adaptation. (Figs. 179, 239.) 



Method of Evolution. A synoptic view of some of the essen- 

 tial facts, presented from a different angle, may serve to clarify 

 our view of evolution as fundamentally a complex problem in 

 genetics. 



In the first place, we have seen that though variations are the 

 rule and not the exception, some are of importance for evolution 

 and some are not. All the evidence indicates that the effective 

 variations are germinal and not somatic. Changes arising in the 

 soma — modifications — are unable to attain representation in the 

 germ so that they are 'born again,' although modifications re- 

 newed by the soma in each generation may enable a race to survive 

 until appropriate recombinations or mutations appear. Evolution 

 must be brought about by changes in the germinal complex — by 

 the evolution of the germ plasm itself. Accordingly selection must 

 operate to eliminate the unfit germ plasm (genotype) rather than 

 the unfit soma (phenotype), though as a matter of fact the fitness 

 of an individual is determined largely by its somatic characters. 

 Dominant genes are directly within the reach of natural selection, 

 whereas recessive genes may slip by because they are frequently 

 concealed by dominants. The latter is a much more select, and 

 selected, group: recessives may be the "skeleton in the nuclear 

 cupboard of the race." We know that inbreeding, e.g., cousin- 

 marriages in man, often reveals recessives because relatives are 

 likely to carry the same recessives and so afford more chance for 

 them to meet and become expressed in the offspring. This presents 

 a complication of the mental picture of the operations of selection 

 which did not exist before our modern concept of phenotype and 

 genotype. Since individuals frequently belie their genotypic con- 

 dition — what they can pass on to their progeny — natural selec- 

 tion has, so to speak, a more devious though not less sure path. 

 (Fig. 182.) 



Secondly, how does the germ plasm change? It will be recalled 

 that germinal alterations result from the usual processes of re- 

 combination and crossing-over, as well as from the more radical 

 mutations — chromosomal aberrations and intrinsic gene changes. 



