THE BREAKDOWN OF CONTINUITY 



and fertile. Yet there must be an overall super-balance between 

 sequences, because selection tests have shown that flies heterory^gous 

 for two alternative sequences have an advantage over others homo- 

 zygous in this respect, but heterozygous for at least part of the genie 

 differences that occur within the same sequence in a strain or in 

 the species at large. This balance of advantage between sequences 

 changes with temperature. 



In pseudoobscura the various sequences float in the population, 

 partly in a heterozygous and partly in a homozygous condition. A 

 number of them may be picked up at any time in any group of 

 individuals. Evidently their super-balance in respect to one another 

 is still more effective in leading to the adjustment of the phenotype, 

 than is the relational balance of the combinations within any one 

 sequence. This is to be expected in a cross-breeding population in 

 the early history of sequential variation, for the inversions will be 

 pegging the combinations which as wholes work well in the 

 heterozygous condition. A further development may, however, be 

 envisaged. 



When sufficient variation is present within sequences to give a 

 good relational balance without sequential heterozygosity, genie 

 adjustment to any permanent change in environmental circum- 

 stances will go on within sequential types rather than between 

 them. Within a sequential type the adjustment of combinations can 

 go on by the recombination of their genes ; but different sequences 

 are genetically isolated from one another (Fig. 79). Thus each 

 sequence may become better fitted than its alternatives for a 

 particular environment. Any further pressure of selection wiU 

 therefore tend to establish a sequence in one or more populations. 

 The sequence will cease to be floating in these populations, and will 

 become, with its derivatives, fixed, uniformly and exclusively 

 homozygous; though at the same time it may continue to float in 

 still other populations. This process appears to be in progress 

 today in pseudoobscura, for although several sequences may be 

 found in any one population, different populations are characterized 

 by particular groups of sequences. 



The three species we have mentioned show the last stage of the 

 process. All three have third chromosomes descended from a 

 common type, but the line of development of miranda differs from 



314 



