202 



DISCOVERY 



in Germany, De Vries in Holland, and Morgan in 

 America, his principles have been shown to hold 

 good for every sort and kind of characters, in every 

 sort and kind of organism, including man himself. 



In the next section we shall see how the most recent 

 work is combining the facts drawn from a study of 

 the chromosomes with those connected with inheritance 

 by unit-factors. 



(3) The Chromosomes as the Bearers of the 

 Unit-Factors 



In our first two sections we were dealing with facts 

 and principles which, though once the subject of 

 heated dispute, are now accepted as proven by all 

 biologists. In the present one, we are touching on 

 discoveries so recent as to be still in the region of 

 controversy. The principles here set forth, however, 

 though still not accepted by many men of science, are 

 rapidly gaining ground, and are being used as a basis 

 by the majority of research workers in heredity. The 

 WTiter believes them to be true in all essentials, and 

 certainly to be far better as a working hypothesis to 

 guide our future advance than any others now in the 

 field, and therefore offers no apology for presenting 

 them in a popular article. 



In our study of unit-factors, we saw that one of 

 Mendel's laws asserted that unit-factors were inherited 

 independently of each other. This was the original 

 form of the law, and is true for many factors. But 

 as animals were bred on a large scale, and single 

 species were worked on in an intensive way, to studj' 

 the relation of a great many separate factors to each 

 other, exceptions were found which have made it 

 necessary to restate the law. In its new form, it runs 

 as follows : (i) Unit-factors are inherited in groups. 

 {2) These groups of factors are not inherited entirely 

 en bloc, but, though the factors within each group are 

 linked together, the linkage is not absolute. In other 

 words, the factors of one group tend to be linked 

 together in inheritance ; but the linkage is not complete, 

 and is of various intensities for different factors. 

 (3) The number of groups is constant for any par- 

 ticular species. 



Into the details of the second of the three points 

 we will not now enter. It is the fact that a constant 

 number of independent groups of factors do exist (and 

 this is undoubted) which interests us for the moment. 



It will be remembered that obser\-ation with the 

 microscope revealed the fact that a constant number 

 of chromosomes was also present in each species. It 

 is, therefore, a natural question to ask whether there 

 is an}' relation between the number of groups and the 

 number of chromosomes. So far, on account of the 

 enormous amount of time and labour involved in 

 carrying out the requisite number of breeding experi- 



ments, the question has only been actually answered 

 for one species, though researches with the same end 

 in view are in progress on many animals and plants. 

 The one form where a definite answer has been obtained 

 is a tiny fly called Drosophila. This is particularly 

 suitable for breeding experiments, as it can be easily 

 kept ; it has large broods of offspring, and a new 

 generation appears about once a fortnight. Morgan 

 and his fellow-workers have now records of about 

 half a million of these flies, all with known pedigree. 

 To obtain a record of the same number of generations 

 in man, we should have to go back before the dawn 

 of history. Something hke two hundred variations, 

 or mutations, have cropped up spontaneously in his 

 stock, and the inheritance of most of these has been 

 worked out. The result has been to show that there 

 are four groups of factors, which is the same number 

 as the number of pairs of chromosomes. Further, the 

 number of factors so far found in each group is roughly 

 proportional to the length of the different chromosomes 

 as seen under the microscope. Morgan and many 

 others, therefore, conclude that the physical basis 

 for the imit-factors is to be found in the chromosomes. 

 If one single factor were to be found not linked to any 

 of the four existing groups, the whole theory would 

 have to be given up, as it would be impossible to 

 reconcile five factor-groups with four pairs of chromo- 

 somes. 



Other facts (connected with linkage) indicate strongly 

 that the unit-factors are arranged in a row along the 



Dingram of the chr 

 mcnt o( uuit-facto 



chromosomes, like beads on a necklace. In this, there- 

 fore, we have an explanation of the facts observed in 

 the division of chromosomes (see the first section). 

 It is, therefore, extremely probable that there are 



