22 THE VARIATION OF ANIMALS IN NATURE 



stringently selected strains under thoroughly controlled con- 

 ditions. It appears by no means impossible that a certain, 

 not altogether negligible, range of variation might remain 

 under the most severe precautions. The complex organisa- 

 tion of the higher animals would appear to be inherently 

 unstable and liable to irreversible changes. The data with 

 regard to conditioned reflexes suggest that this may be the 

 case in the nervous system and it is likely that other 

 organ-systems may be liable to similar ' habit-formation.' 

 Under severely controlled conditions it might still be possible 

 for permanent ' deformations ' to result from intrinsic causes. 

 There are, of course, good grounds for believing that 

 physiological rhythms may be permanent in at least the 

 lifetime of the individual. Thus Payne (1931) found that 

 in the parasitic wasp Microbracon hebetor, adults taken from 

 cultures reared at high temperatures lived a shorter time at all 

 temperatures than those taken from lower temperatures. In 

 the future it may be hoped that the large amount of research 

 now being conducted into the effects of controlled conditions 

 of temperature and humidity on insects will provide significant 

 data. 



2. The Basis of Heritable Variation 



The nature and distribution in heredity of the visible 

 characters of an organism are to an important extent deter- 

 mined by the way in which they are represented in the 

 chromosomes of the germ cells. Thus some characters are 

 determined by a single gene, others by several genes, and others 

 again by complementary genes. Or again the distribution of 

 certain characters will depend on whether linkage occurs or 

 not. The way in which characters are genetically determined 

 will thus influence their variation. 



In discussing the origin of variation we have to distinguish 

 carefully between the origin of new hereditary material and 

 the occurrence of variation due to differences in the way in 

 which characters are genetically represented. The latter 

 includes, for example, the effects of recombinations and com- 

 plementary genes. We have, therefore, to examine the 

 various ways in which characters are genetically determined 

 in order to distinguish the sources of new evolutionary steps 

 (mutation) from other forms of hereditary variation. 



