538. THE PURE LINE HYPOTHESIS. 



It must be ever remembered that referring: heredity to a 

 system of factors, determinants, centres of force, memories, etc., 

 in the germ-cell cannot explain heredity ; it may certainly assist 

 in predicting- results, but a real explanation must include 5 know- 

 ledge as to how the agencies produce the eft'ect. Such know- 

 ledge is unattainable, as it would necessitate an understanding 

 of the nature of life, consciousness and memory. 



As a simple example of blending inheritance we may take 

 the amount of spotting on the lower surface of the flower of the 

 foxglove. A plant with flowers having very few spots was 

 crossed with a plant in which the flowers were richly spotted. 

 The offspring carried flowers which tended to be inter- 

 mediate between the two parents in the amount of spotting. It 

 was previously ascertained that the two parents were gametically 

 pure with regard to this character. Each was self-fertilised, 

 and the offspring of each were similar to the parent. 



The oft'spring of the cross were self-fertilised and families 

 were raised. No segregation occurred, the individuals of each 

 family tended to resemble the parent, and it w^as totally im- 

 possible to separate the plants into two groups, one resembling 

 the female grandparent and the other the male grandparent. 

 Another generation was raised by self -fertilisation, and although 

 there was considerable variation in the amount of spotting, yet 

 it may be very definitely affirmed that no segregation into the 

 two great grandparental types occurred. The simplicitv of the 

 character is advantageous, since the inheritance of any compli- 

 cated character, such as the stature of man, leaves room for 

 much ingenious speculation in trying to interpret it in Mendelian 

 terms. 



It would appear that, according to the factorial hypothesis, 

 we must suppose a factor for a small amount of spotting occurred 

 in a chromosome of one parent and a factor for much spotting 

 was present in the other parent. On crossing, these two factors 

 would be present in the contiguous chromosomes of an homo- 

 logous pair. We must then suppose that the two factors mutuallv 

 influence each other, and thereby become quite similar to each 

 other, and intermediate in nature between the two original 

 factors. It is doubtful, however, whether this interpretation 

 w^ould be welcomed by the strict Mendelian. (See plate.) 



Obvious Mendelian relationships only occur in the inheri- 

 tance of individuals which have resulted from the crossing of 

 forms which bear characters that refuse to mingle intimatelv 

 with each other. The laws relating to this kind of hybridism 

 must not be allowed to obscure the relatively more important 

 type of inheritance where the characters of parents blend, and 

 there is no subsequent tendency for the separation of the two 

 characters. In the opinion of the writer, this latter tvpe of in- 

 heritance is much more intimately associated with the evolu- 

 tionary process than the former. 



It is, in a sense, unsatisfactorv to be obliged to recognise 

 two types of heredity — namely, non-blending and blending inheri- 



