470 AN INTRODUCTION TO ZOOLOGY 



one of the extreme variants of this subsidiary type were taken, its 

 offspring showed regression to the type of the line not the type of 

 the species. He denned as a " pure line," therefore, a group of 

 individuals which has a normal variability of its own, and the off- 

 spring of which by self-fertilisation breed true to the type of their 

 own particular group. A result of this is to show that selection in 

 a population will come to an end when you have been able to 

 separate out the most eccentric line. Further progress must be 

 made by utilising mutations or dealing with the characters in a 

 Mendelian manner. This phenomenon of a pure line is of consider- 

 able practical importance to the breeder of plants. 



To return again to Mendel's work, we find that we can deduce 

 from his and similar experiments two general principles. The first 

 is, that whatever unit factors may be introduced into the zygote at 

 fertilisation, such factors are not completely blended, but are sorted 

 out again in the gametes it produces. This is termed the law of 

 segregation. The second is, as we have seen when considering the 

 behaviour of two pairs of unit characters, that the inheritance of each 

 pair of characters is independent of the other. This is known as the 

 law of independent assortment. As was noted above, the rediscovery 

 of Mendel's work served as an inspiration to further investigation 

 along similar lines, and many fruitful fields of research were opened 

 up. One of these that is particularly associated with the names of 

 Morgan and his fellow- workers is of interest, sinee it is in the main 

 a direct extension of Mendel's own experimentation, and has yielded 

 additional laws. In the first place, it has been found that the second 

 law given above is not of universal application, and that, while it is 

 true in quite a number of cases, an ever-increasing number of examples 

 are found in which there is a marked tendency for groups of cha- 

 racters to be inherited together from generation to generation. This 

 tendency to keep together, so that it is really the groups that are 

 independently assorted rather than the individual characters, is 

 termed linkage, and it is obvious that this limits the law of inde- 

 pendent assortment. There are certain theoretical points that can 

 be advanced to explain this apparent contradiction. We have 

 reason for thinking that the chromosomes are the actual bearers of 

 the factors whose presence in the zygote is necessary for the appear- 

 ance of certain characters in the adult. Moreover, the chromosomes 

 retain their identity from division to division, and the factors are 

 related to them in a very intimate and individual manner, and not 

 in a loose general way. We should expect, therefore, that the 

 character groups correspond with the chromosomes. In support 

 of this, we find that in Drosophila melanogaster , the vinegar fly, 

 there are as many groups as chromosomes, the three largest con- 



