S04 ZOOLOGY 



7. At this point a most interesting question arises. If a 

 single chromosome carries many genes and at division splits all 

 of them, as has been indicated, how does it come about that 

 qualities determined by this chromosome, when once brought 

 together, are not perpetually linked? How could the unit char- 

 acters ever be combined in any different way? It is believed 

 that two homologous genes or groups of genes may "cross-over" 

 during the pairing of the chromosomes preceding the reduction 

 division. That is to say that exchanges of genes may take place 

 between the chromosomes, each gene coming to reside in the 

 other member of the pair. It is thought that this shuffling of 

 the genes by the combination of linkage and crossing over, 

 helps to account for the various permutations of unit qualities 

 seen in breeding. It will be seen that this gives rise to no new 

 qualities. 



8. Much evidence has been accumulated pointing to the 

 conclusion that maleness or femaleness (sex) is inherited very 

 much as other unit characters, and is determined by one or more 

 special chromosomes or by certain genes in these. Quite a 

 number of sex-linked characters have been discovered. 



495. Isolation. — When new variations occur it is clear that 

 they will have a better chance to accumulate and result in a 

 new and permanent stock if for any reason these new cliaracters 

 cannot be "swamped" or buried by crossing with the more 

 numerous and conservative members of the parent stock. Re- 

 cessive qualities could be prevalent only through such isolation. 



The effects of geographic or physical isolation is seen on is- 

 lands which have been populated from the mainlands, or in 

 neighboring lakes, or in valleys between which there are barrier 

 mountains. In such conditions the animals and plants are dis- 

 tinctly different from the species from which they are believed 

 to have sprung, and those on different islands are different from 

 one another. It is believed by many observers that a larger 

 number of related species is found under such conditions than 

 where there is free interbreeding and migration. 



It is possible also that there are internal hindrances to mating 

 which would operate in much the same way as geographic iso- 

 lation. There may arise some instinctive feeling which would 



