58 



NATURE 



[January 12, 1922 



complex example is given by crossing black polled 

 with red horned cattle, which in the first generation 

 yields all black polled animals, but in the second 

 generation a very mixed progeny arises. If the 

 factors for the black-red and the poUed-horned pairs 

 are transmitted in the same manner, but indepen- 

 dently of one another, then the second generation will 

 consist of four classes : black-polled, black-horned, 

 red-polled, and red-horned in the ratio of 9 : 3 : 3 : i. 

 This ratio has not been verified on a comprehensive 

 scale for the cattle cross, but it has been worked out 

 in all details in several cases for smaller animals. 

 That horned-blacks and poUed-reds appear in the 

 second filial generation means that there has been a 

 "break up" of the parental tyjies, and the new 

 classes arise through re-combination of the two 

 pairs of factors in which the original parents 

 differed. 



Many of the characters of animals owe their mani- 

 festation to the presence of one or other definite 

 factor transmitted according to a definite scheme. If 

 these factors are not divisible under normal conditions 

 they must be transmitted through the germ-cells as 

 definite entities producing their full effect in each suc- 

 cessive generation. Therefore, if these factors are 

 relativelv permanent, arid follow a fixed scheme of 

 distribution in heredity, it is obvious that the 

 characters of living things can be brought under 

 accurate control by the breeder. This factorial theory 

 of heredity has been tested and proved to hold -good 

 in a large number of cases, and the problem now 

 engaging the attention of research workers at Cam- 

 bridge is to find whether it can be applied to those 

 cases where at first sight there appears no suggestion 

 of clear-cut alternative pairs of characters. 



The last two papers of the series under notice deal 

 with some of the experiments carried out on these 

 lines. One of the most extensive analyses was 

 designed to investigate the inheritance of weight in 

 poultry. Two standard breeds were chosen ; for the 

 larger bird the Gold-pencilled Hamburgh, and for the 

 smaller one the Silver Sebright Bantam, the latter 

 being, roughly, three-fifths of the weight of the 

 former. The first-cross birds were intermediate in 

 size, but in the second generation there was a very 

 wide variation. The majority of birds were between 

 the weights of the original parental birds, but a few 

 were larger than the Hamburgh, while a few were 

 smaller than the Sebright. Nilsson-Ehle, working 

 on wheat and oats, was the first to give an explana- 

 tion of such cases, and the closeness with which the 

 theory fitted his results left little doubt of its being 

 a true interpretation. Essentially, his theorv is that 

 a similar effect may be brought about by more than 

 one factor, though such factors are independentlv 

 transmitted. Accordingly, if there are several similar 

 factors, A, B, C, D, etc., which influence the weight 



of poultry, then a bird possessing none of these factors 

 will be the smallest type. When it contains A, it 

 will be rather larger; when it contains A and B, it 

 will be larger again, and so on until the largest bird 

 is reached which contains the full collection of the 

 weight factors. 



This theory was found to cover all the observed 

 facts, and although it is not suggested that weight is 

 dependent solely upon such factors, yet it seems 

 probable that even such complicated characters can 

 be interpreted in terms of definite factors. On the 

 other hand, very different results were obtained in 

 experiments on rabbits, where the large Flemish was 

 crossed with the small Polish rabbit. In this case the 

 F2 generation contained no animals at all approaching 

 the size of the original Flemish, and no explanation 

 of this can at present be offered. Further experi- 

 ments on rabbits were concerned with the inheritance 

 of coat patterns, and the analysis of the continuous 

 series from self-colour to almost white provided an 

 interpretation in terms of the factorial theory. 



Another interesting series of analyses dealt with 

 the peculiar forni of inheritance known as sex-linked 

 heredity. This can be illustrated by the Hamburgh- 

 Sebright cross used for the weight experiments. The 

 Hamburgh was a gold-pencilled and the Sebright a 

 silver, and the experimental work showed that silver 

 and gold form an alternative pair, silver being a 

 simple dominant to gold, but in the hen the trans- 

 mission of the factor for silver is sex-linked. The 

 silver hen is never pure for the silver factor ; half 

 of her eggs are "silver " and half are "gold"; more- 

 over, she transmits the silver factors to her male- 

 producing eggs and the gold to her female-producing 

 eggs. A large number of birds have been bred .from 

 the mating of silver hen and gold cockerel, but there 

 has not been one exception to the rule that the 

 cockerels all come silver and the pullets all gold. 

 This sex-linked type of inheritance is found in several 

 other characters iVi poultry, and it may prove of 

 economic importance, for by making use of suitable 

 crosses the breeder of poultry for egg-production can 

 be sure of rearing nothing but pullets through the 

 earlier, and more costly, stages. 



Further experiments dealt with the Inheritance by 

 cocks of hennv feathering, while others were con- 

 cerned with the characters of egg-colour and broodl- 

 ness In poultry. These had to be curtailed con- 

 slderablv owing to war conditions, although some 

 interesting results were obtained. 



Although all these analyses may prove to be of 

 economic value, yet it must be remembered that the 

 j "main object of the work at Cambridge Is the 

 I elucidation of the nrincioles that underlie the pheno- 

 j mena of hereditv," and when these have been re- 

 j vealed the application can be left to those who wll! 

 I derive profit from it. 



A Petrological Microscope. 



"XITE have received for examination from Messrs. R. 

 *'' and J. Beck an example of their "Standard 

 London Petrological Microscope," which they have 

 recently designed for the use of students. It embodies 

 some of the recommendations of a committee of the 

 British Science Guild, which carefully considered the 

 subject (Journal of the British Science Guild, 

 November, 1916, pp. 28-30). The microscope, which 

 is strongly built and stands firmly, has the following 

 distinctive features : 



The analyser is a form of the Abbe prism, devised 

 NO. 2724, VOL. IO9J 



by Mr. E. M. Nelson. It is placed Immediately be- 

 low the upper lens of the ocular, and slides laterally 

 in and out of position. This arrangement, while it 

 does not appreciably contract the field, has the ad- 

 vantage that it allows a quartz wedge to be inserted 

 between the nicols in the focus of the ocular, with 

 the result that the colour bands are sharply defined, 

 as Is also the dark band indicating the position of 

 compensation. It is stated that in certain circum- 

 stances a faint second image of the cross wires can 

 be seen, but it Is scarcely noticeable, and causes 



