K. — BOTANY. 171 



century, for in 1900 was announced the rediscovery of Mendel's work, 

 actually given to the world thirty-live years earlier, but at the time 

 leaving no impress upon scientific thought. The story of the Austrian 

 monk and the details of his experiments earned out in the monastery 

 garden upon races of the edible pea are now familiar history, and 1 

 need not recount them here. Having formed the idea that in order 

 to arrive at a clearer understanding of the relation of organisms to 

 their progeny the problem must be studied in its simplest form, Mendel 

 came to see that a scheme of analysis must deal not with mass popula- 

 tions but with a smaller unit — the family, and that each character of 

 the individual must be separately investigated. 



Selecting for his experiments races v/hich showed themselves to be 

 pure-breeding and mating together those exhibiting chai'acters of such 

 opposite nature as to constitute a pair — e.g., tall with short, yellow- 

 seeded with green-seeded— he obtained results which could be accounted 

 for if it were supposed that these opposite, or as we should now term 

 them allelomorphic, characters were distributed unaltered and in equal 

 profortion to the reproductive cells of the cross-bred organism. It is 

 this conception of the pure nature of the germ-cells, irrespective of 

 whether the oi'ganism forming them be of pure-bred or cross-bred 

 descent, which revolutionised our conceptions of Heredity and laid the 

 foundations upon which we build to-day. For the intervening years 

 have seen the instances in which the Mendelian theory is found to 

 hold mount steadily from day to day, furnishing a weight of evidence 

 in its support which is incontrovertible. 



It chanced that in each pair of characters selected by Mendel for 

 experiment the opposites are related to each other in the following 

 simple manner: An individual which had received both allelomorphs, 

 one from either parent, exhibited one of the two characteristics, hence 

 called the dominant, to the exclusion of the other. Among the offspring 

 of such an individual both characteristics appeared, the dominant in 

 some, its opposite, the recessive, in others, in the proportion approxi- 

 mately of three to one. This is the result which might be expected 

 from random pairing in fertilisation of two opposites, where the mani- 

 festation in the zygote of the one completely masks the presence of the 

 other. As workers along Mendelian lines increased and the field of 

 inquiry widened, it soon, however, became apparent that the dominant- 

 recessive relationship is not of universal occurrence. It likewise 

 became clear that the simple ratios which obtained in Mendel's experi- 

 ments are not characteristic of every case. Mendel's own results were 

 all, as it happened, explicable on the supposition that the two alterna- 

 tive forms of each character were dependent on a single element or 

 factor. By a fortunate accident none of ,the complex factorial inter- 

 relations which have since been brought to light in other cases obscured 

 the expression in its simplest form of the results of germ purity. It 

 is our task, in the light of ,this guiding principle, to attempt to elucidate 

 these more complicated types of inheritance. 



We now know, for example, that many characters are not con- 

 trolled by one single factor, but by two or more. One of the most 

 familiar instances of the two-factor character is the appearance of the 



