S4 



SCIENTIFIC AGRICULTURE 



Xrj\ember, 19l'l 



lieritanee of humau stature, he was enabled 

 to observe that the offspring of j)arents 

 either taller or shoiter than the average of 

 the race tended in a certain degree to re- 

 tain the parental eharacteristic The es- 

 sential point of Galton's work in this c(>n- 

 nection is that it was not only in harmony 

 with the selection hypothesis current at 

 that time, but it was also regarded as af- 

 fording a mathematical measure of the 

 amount of progress tliat could be made. 



In 18H3, two years after Darwin's 

 death, Weismann, a German zoologist, 

 brought forAvard a iieAv theory of heredity. 

 According to the Wei.smannian concept, 

 vai-iations as regards their transmissabi- 

 lity fell into two well-defined categories, 

 first, those Avhich had a basis in the germ- 

 l)lasm of the organism and passed through 

 it from generation to generation, and se- 

 condh' those modifications that were due 

 l)urely to the environment whose effect 

 was upon the body only and consequently 

 could not be transmitted to the offs])ring. 

 Weismann, further, advanced the theory 

 that the chromosomes were the essential 

 ])hysical bodies within the germ cells con- 

 cerned in heredity. Added weight to this 

 view was provided by the reseaiches of 

 Boveri and other cytologists who showed 

 that in certain animal forms the germ cells 

 are laid down at an early stage in tlie de- 

 velopment of the organism and have theii- 

 origin in a part of the embryo different 

 fi-om that giving rise to the body. 



The epoch-making researches of Gregoi: 

 Mendel brought to light in the year 1900 

 proved to be the long sought master key 

 to inheritance. The work of the great 

 Augustinian monk will stand for all time 

 as a model of that type of constructive 

 effort which mankind has been too prone 

 to discourage. It is not necessai-y here to 

 i-ecount Mendel's clear cut expciiments 

 with the garden i)ea. his careful analysis 

 of the breeding facts he obtained and his 

 fitting the theory thereto that we now 

 know as Mendel's Laws of Heredity. In 

 visualizing his results Mendel used an 

 algebraic notation, the hybrids obtained on 

 ci-ossing a plant ])ure for the dominant 

 character A with one pure for the reces- 

 sive V;. being represented as .having the 

 composition A(7. And il shoidd be noted 

 that he made the i:' ■ncral in;i1 '.(■iwiti.'nl 



deduction that iu any generation n, the 

 ratio of this pair Art would be 2" — 1 A : 

 2 Art : 2" — Ifl, assuming equal fertility in 

 all the plants and constant self-fertiiiza- 

 tion throughout. As shall be presently 

 seen tliis was an important contribution to 

 the solution of the selection problem. The 

 further elucidation of this important ques- 

 tion was a direct i-esult of the impetus af- 

 forded the study of heredity hy Mendel's 

 work. 



Johannsen's ' classical experiments 

 with the common bean led to a discovery 

 of tremendous significance to breeding and 

 gave to us our first real knowledge regard- 

 ing the genetic composition of self-fer- 

 tilized species. His results, moreover, ex- 

 plained the conflicting results of those 

 many plant breeders who iu the years be- 

 fore liad been practising selection iu the 

 vain hope that progress could be made 

 thereby, iri'esi)ective of the pollination 

 liabit of the plant. 



The Danish botanist's early results led 

 him to doubt the validity of (Talton's law. 

 Selecting seeds of different >izes and of 

 known weights for planting, he harvested 

 and weighed the progeny of each plant se- 

 parately and observed that instead of the 

 2/3 inheritance that Galton found in size 

 of seed in sweet pea and in stature of man, 

 the average amount of resemblance to 

 parental types was about 14 • He noted 

 also tluit the offspring of similar mother 

 beans differed very markedly in size. The 

 frequency distribution of the progeny of 

 parent beans of a given wciglit proved to 

 be skew, a result contrary to expectation 

 if the material belonged to a single type. 

 With this indication of heterogeneity, 

 •lohannsen ])ursued the problem further. 

 In 1901 he harvested the .seeds of 19 plants 

 each of which originated from a single seed 

 chosen from the previous year's crop. Of 

 the 1901 crop 524 seeds were planted re- 

 presenting, respectively, each of the 19 

 lines. In harvesting the 1902 crop each line 

 and each plant was kept distinct so that 

 the lineage of every single bean weighed 

 was known precisely. By tabulatiujr in 

 juxtai)osition the weights nf the mother 

 beans and offspring in each nf ftic 19 lines 



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